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AS_publications2019-21.txt
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Title: A machine learning approach for classification of accretion states of black hole binaries
Authors: Sreehari, H., & Nandi, A.
Bibliographic Code: 2021MNRAS.502.1334S
Keywords: accretion, accretion disc, black hole physics, methods: data analysis, X-rays: binaries, Astrophysics - High Energy Astrophysical Phenomena
Abstract: In this paper, we employ Machine Learning algorithms on multimission observations for the classification of accretion states of outbursting black hole X-ray binaries for the first time. Archival data from RXTE, Swift, MAXI, and AstroSat observatories are used to generate the hardness intensity diagrams (HIDs) for outbursts of the sources XTE J1859+226 (1999 outburst), GX 339-4 (2002, 2004, 2007, and 2010 outbursts), IGR J17091-3624 (2016 outburst), and MAXI J1535-571 (2017 outburst). Based on variation of X-ray flux, hardness ratios, presence of various types of quasi-periodic oscillations (QPOs), photon indices, and disc temperature, we apply clustering algorithms like K-Means clustering and Hierarchical clustering to classify the accretion states (clusters) of each outburst. As multiple parameters are involved in the classification process, we show that clustering algorithms club together the observations of similar characteristics more efficiently than the 'standard' method of classification. We also infer that K-Means clustering provides more reliable results than Hierarchical clustering. We demonstrate the importance of the classification based on machine learning by comparing it with results from 'standard' classification.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2021MNRAS.502.1334S">2021MNRAS.502.1334S</a>
Title: Ultraviolet Imaging Telescope (UVIT) observation of the Galactic globular cluster NGC 7492
Authors: Kumar, R., Pradhan, A. C., Mohapatra, A., Moharana, A., Ojha, D. K., Parthasarathy, M., & Murthy, J.
Bibliographic Code: 2021MNRAS.502..313K
Keywords: stars: evolution, Hertzsprung-Russell and colour-magnitude diagrams, stars: horizontal branch, globular clusters: individual: NGC 7492, ultraviolet: stars, Astrophysics - Astrophysics of Galaxies, Astrophysics - Solar and Stellar Astrophysics
Abstract: We present detailed photometric observations of the Galactic globular cluster NGC 7492 using the data obtained with two far-ultraviolet (FUV; 1300-1800 Å) and three near-ultraviolet (NUV; 2000-3000 Å) filters of Ultraviolet Imaging Telescope (UVIT) onboard the AstroSat satellite. We confirmed the cluster membership of the extracted sources using Gaia data release 2 (Gaia DR2) proper motion data. We have used colour-magnitude diagrams (CMDs) using UVIT and GAIA filters to separate out different evolutionary stages of the stars present in the cluster. We have identified a new extreme horizontal branch (EHB) star at the core of the cluster using UV and UV-optical CMDs. The estimated distance modulus of the cluster is 16.95 ± 0.05 obtained by fitting BaSTI isochrones with cluster parameters, [Fe/H] = -1.8 dex and age =12.0 Gyr on the V - I versus V CMD. Interestingly, only the EHB star and blue horizontal branch stars (BHBs) among the UV-bright hot sources are detected in FUV filters of UVIT. We have derived the effective temperature of BHBs using colour-temperature relation and spectral energy distributions (SEDs) of multiband filters, which are in the range from 8000 to 10 500 K. We find a variation of He abundance of BHBs by fitting the BaSTI ZAHB (zero-age horizontal branch). The range in the He abundance of the BHBs corresponding to the best-fitting isochrones is from 0.247 to 0.350. We have estimated various physical parameters of the newly identified EHB star in the cluster using SED fit and post-HB evolutionary tracks. We have studied the radial distribution of all the sources of the cluster detected in UVIT. The sources detected in FUV filters extend beyond the half-light radius (1.15 arcmin) of the cluster, whereas the sources detected in NUV filters extend beyond the tidal radius (9.2 arcmin) of the cluster.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2021MNRAS.502..313K">2021MNRAS.502..313K</a>
Title: AstroSat view of IGR J17091-3624 and GRS 1915 + 105: decoding the 'pulse' in the 'Heartbeat State'
Authors: Katoch, T., Baby, B. E., Nandi, A., Agrawal, V. K., Antia, H. M., & Mukerjee, K.
Bibliographic Code: 2021MNRAS.501.6123K
Keywords: accretion, accretion discs, black hole physics, stars: individual: IGR J17091-3624, GRS 1915 + 105, X-rays: binaries, Astrophysics - High Energy Astrophysical Phenomena
Abstract: IGR J17091-3624 is a transient galactic black hole which has a distinct quasi-periodic variability known as 'heartbeat', similar to the one observed in GRS 1915 + 105. In this paper, we report the results of ˜125 ks AstroSat observations of this source during the 2016 outburst. For the first time, a double-peaked QPO (DPQ) is detected in a few time segments of this source with a difference of δf ˜ 12 mHz between the two peaks. The nature of the DPQ was studied based on hardness ratios and using the static as well as the dynamic power spectrum. Additionally, a low-frequency (25-48 mHz) 'heartbeat' single-peak QPO (SPQ) was observed at different intervals of time along with harmonics (50-95 mHz). Broad-band spectra in the range 0.7-23 keV, obtained with Soft X-ray Telescope and Large Area X-ray Proportional Counter, could be fitted well with combination of a thermal Comptonization and a multicolour disc component model. During AstroSat observation, the source was in the soft-intermediate state (SIMS) as observed with Swift/XRT. We present a comparative study of the 'heartbeat' state variability in IGR J17091-3624 with GRS 1915 + 105. Significant difference in the timing properties is observed although spectral parameters (Γ ˜ 2.1-2.4 and T<SUB>max</SUB> ˜ 0.6-0.8 keV) in the broad energy band remain similar. Spectral properties of segments exhibiting SPQ and DPQ are further studied using simple phase-resolved spectroscopy which does not show a significant difference. Based on the model parameters, we obtain the maximum ratio of mass accretion rate in GRS 1915 + 105 to that in IGR J17091-3624 as ˜25: 1. We discuss the implications of our findings and comment on the physical origin of these exotic variabilities.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2021MNRAS.501.6123K">2021MNRAS.501.6123K</a>
Title: Broad-band 'spectro-temporal' features of extragalactic black hole binaries LMC X-1 and LMC X-3: an AstroSat perspective
Authors: Bhuvana, G. R., Radhika, D., Agrawal, V. K., Mandal, S., & Nandi, A.
Bibliographic Code: 2021MNRAS.501.5457B
Keywords: accretion, acretion discs, black hole physics, radiation mechanisms: general, stars: black holes, stars: individual: LMC X-1 and LMC X-3, X-rays: binaries
Abstract: We present the first results of extragalactic black hole (BH) X-ray binaries LMC X-1 and LMC X-3 using all the archival and legacy observations by AstroSat during the period of 2016-2020. Broad-band energy spectra (0.5-20 keV) of both sources obtained from the Soft X-ray Telescope and Large Area X-ray Proportional Counter on-board AstroSat are characterized by strong thermal disc blackbody component (kT<SUB>in</SUB> ∼ 1keV, $f_{\mathrm{ disc}}\ 79{{\ \rm per\ cent}}$ ) along with a steep power-law (Γ ∼ 2.4-3.2). Bolometric luminosity of LMC X-1 varies from $7{\,\mathrm{ to}\,}10{{\ \rm per\ cent}}$ of Eddinon luminosity (L<SUB>Edd</SUB>) and for LMC X-3 is in the range $7{-}13{{\ \rm per\ cent}}$ of L<SUB>Edd</SUB>. We study the long-term variation of light curve using MAXI data and find the fractional variance to be ${\sim}25{{\ \rm per\ cent}}$ for LMC X-1 and ${\sim}53{{\ \rm per\ cent}}$ for LMC X-3. We examine the temporal properties of both sources and obtain fractional rms variability of power density spectrum in the frequency range 0.002-10 Hz to be ${\sim}9{}-17{{\ \rm per\ cent}}$ for LMC X-1, and ${\sim}7{}-11{{\ \rm per\ cent}}$ for LMC X-3. The 'spectro-temporal' properties indicate both sources are in thermally dominated soft state. By modelling the spectra with relativistic accretion disc model, we determine the mass of LMC X-1 and LMC X-3 in the range $7.64{-}10.00$ and $5.35{-}6.22\, \mathrm{ M}_{\odot }$ , respectively. We also constrain the spin of LMC X-1 to be in the range 0.82-0.92 and that of LMC X-3 in 0.22-0.41 with 90 per cent confidence. We discuss the implications of our results in the context of accretion dynamics around the BH binaries and compare it with the previous findings of both sources.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2021MNRAS.501.5457B">2021MNRAS.501.5457B</a>
Title: Thermonuclear X-ray bursts detected in Cyg X-2 using AstroSat/LAXPC
Authors: Devasia, J., Raman, G., & Paul, B.
Bibliographic Code: 2021NewA...8301479D
Keywords: X-Rays: Binaries, (Stars:) stars: Neutron, Accretion, Accretion discs, Astrophysics - High Energy Astrophysical Phenomena
Abstract: We report the detection of 5 Type-1 thermonuclear X-ray bursts and one burst-like event in the neutron star LMXB source, Cyg X-2 using X-ray data obtained with the Large Area X-ray Proportional Counter (LAXPC) instrument on board AstroSat. We carry out an energy resolved burst profile analysis as well as time resolved spectral analysis for each of the bursts and characterize their properties. All bursts are weak with burst peak-to-persistent intensity ratios < 3, decay times ~ 1.2 s, and with fluences ~ 1 ×10<SUP>-8</SUP> ergs/cm<SUP>2</SUP>, indicating that the observed bursts are Helium fuelled flashes. An evolution of the blackbody temperature and radius is also observed during each burst. We carry out a search for Burst Oscillations (BO) and derive upper limits to the rms fractional amplitude for BO (for all the bursts) to be ~ 1%. We also carried out search for Quasi Periodic Oscillations (QPOs) in the power density spectra and we obtain upper limits to the fractional rms amplitude as ~ 3.4% at frequencies close to ~ 5.6 Hz. We further carry out spectral and timing analysis of the non-burst persistent emission along with a study of the hardness-intensity and colour-colour diagrams. Using results from our analysis we infer that during this observation in 2016, Cyg X-2 can be characterized as being in the early Flaring Branch (FB) with a puffed up accretion disk and a clumpy coronal structure while undergoing medium-to-high levels of accretion.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2021NewA...8301479D">2021NewA...8301479D</a>
Title: UVIT-HST-Gaia-VISTA study of KRON 3 in the Small Magellanic Cloud: A cluster with an extended red clump in UV
Authors: Nayak, P. K., Subramaniam, A., Subramanian, S., Sahu, S., Mondal, C., Cioni, M.-R. L., Bell, C. P. M., Bandyopadhyay, A., & Chung, C.
Bibliographic Code: 2021MNRAS.tmp..441N
Keywords: (galaxies:) Magellanic Clouds, galaxies: star clusters, Astrophysics - Astrophysics of Galaxies, Astrophysics - Solar and Stellar Astrophysics
Abstract: We have demonstrated the advantage of combining multi-wavelength observations, from the ultraviolet (UV) to near-infrared, to study Kron 3, a massive star cluster in the Small Magellanic Cloud. We have estimated the radius of the cluster Kron 3 to be 2 ${_{.}^{\prime}}$ 0 and for the first time, we report the identification of NUV-bright red clump (RC) stars and the extension of the RC in colour and magnitude in the NUV versus (NUV-optical) colour-magnitude diagram (CMD). We found that extension of the RC is an intrinsic property of the cluster and it is not due to contamination of field stars or differential reddening across the field. We studied the spectral energy distribution of the RC stars, and estimated a small range in temperature ∼5000-5500 K, luminosity ∼60-90 L<SUB>⊙</SUB> and radius ∼8.0-11.0 R<SUB>⊙</SUB> supporting their RC nature. The range of UV magnitudes amongst the RC stars (∼23.3 to 24.8 mag) is likely caused by the combined effects of variable mass loss, variation in initial helium abundance (Y<SUB>ini</SUB> = 0.23 to 0.28), and a small variation in age (6.5-7.5 Gyr) and metallicity ([Fe/H] = -1.5 to -1.3). Spectroscopic follow-up observations of RC stars in Kron 3 are necessary to confirm the cause of the extended RC.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2021MNRAS.tmp..441N">2021MNRAS.tmp..441N</a>
Title: Study of UV-bright stellar populations in the globular cluster NGC 1261 using Astrosat
Authors: Rani, S., Pandey, G., Subramaniam, A., Sahu, S., & Rao, N. K.
Bibliographic Code: 2021MNRAS.501.2140R
Keywords: blue stragglers, Hertzsprung-Russell and colour-magnitude diagrams, stars: horizontal branch, globular clusters: individual: NGC 1261, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies
Abstract: We present UV photometry of the globular cluster NGC 1261 using images acquired with the Ultraviolet Imaging Telescope (UVIT) on board Astrosat. We performed point-spread function (PSF) photometry on four near-UV (NUV) and two far-UV (FUV) images and constructed UV colour-magnitude diagrams (CMDs), in combination with the Hubble Space Telescope (HST), Gaia, and ground-based optical photometry for member stars. We detected the full horizontal branch (HB) in the NUV and blue HB in the FUV and identified two extreme HB (EHB) stars. HB stars have a tight sequence in UV-optical CMDs, well fitted with isochrones generated (age 12.6 Gyr, [Fe/H] = -1.27 metallicity) using updated BaSTI-IAC models. Effective temperatures (T<SUB>eff</SUB>), luminosities, and radii of bright HB stars were estimated using the spectral energy distribution. As we detect the complete sample of UV-bright HB stars, the hot end of the HB distribution is found to terminate at the G-jump ( $T_{\rm eff}\, \sim$ 11500 K). The two EHB stars, fitted well with single spectra, have T<SUB>eff</SUB> = 31000 K and a mass = 0.495 M<SUB>⊙</SUB>, and follow the same T<SUB>eff</SUB>-radius relation as the blue HB stars. We constrain the formation pathways of these EHB stars to extreme mass loss in the RGB phase (due either to rotation or enhanced helium) or a early hot-flash scenario.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2021MNRAS.501.2140R">2021MNRAS.501.2140R</a>
Title: The First Extensive Exploration of UV-bright Stars in the Globular Cluster NGC 2808
Authors: Prabhu, D. S., Subramaniam, A., & Sahu, S.
Bibliographic Code: 2021ApJ...908...66P
Keywords: Globular star clusters, Post-asymptotic giant branch stars, Ultraviolet photometry, Hertzsprung Russell diagram, Horizontal branch stars, 656, 2121, 1740, 725, 746, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies
Abstract: In this study, we identified and characterized the hot and luminous UV-bright stars in the globular cluster NGC 2808. We combined data from the Ultra Violet Imaging Telescope (UVIT) on board the Indian space satellite AstroSat with the Hubble Space Telescope UV Globular Cluster Survey data for the central region (within $\sim 2\buildrel{\,\prime}\over{.} 7\times 2\buildrel{\,\prime}\over{.} 7$ ) and Gaia and ground-based optical photometry for the outer parts of the cluster. We constructed the UV and UV-optical color-magnitude diagrams, compared the horizontal branch (HB) members with the theoretical zero- and terminal-age HB models, and identified 34 UV-bright stars. The spectral energy distributions of the UV-bright stars were fitted with theoretical models to estimate their effective temperatures (12,500-100,000 K), radii (0.13-2.2 R<SUB>⊙</SUB>), and luminosities (∼40-3000 L<SUB>⊙</SUB>) for the first time. These stars were then placed on the Hertzsprung-Russell diagram, along with theoretical post-HB evolutionary tracks, to assess their evolutionary status. The models suggest that most of these stars are in the asymptotic giant branch (AGB)-manqué phase, and all except three have evolutionary masses <0.53 M<SUB>⊙</SUB>. We also calculated the theoretically expected number of hot post-(early)-AGB stars in this cluster and found the range to match our observations. Seven UV-bright stars located in the outer region of the cluster, identified from the AstroSat/UVIT images, are ideal candidates for detailed follow-up spectroscopic studies.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2021ApJ...908...66P">2021ApJ...908...66P</a>
Title: A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE). IX. The effects of ram pressure stripping down to the scale of individual HII regions in the dwarf galaxy IC 3476
Authors: Boselli, A., Lupi, A., Epinat, B., Amram, P., Fossati, M., Anderson, J. P., Boissier, S., Boquien, M., Consolandi, G., Côté, P., Cuillandre, J. C., Ferrarese, L., Galbany, L., Gavazzi, G., Gómez-López, J. A., Gwyn, S., Hensler, G., Hutchings, J., Kuncarayakti, H., Longobardi, A., Peng, E. W., Plana, H., Postma, J., Roediger, J., Roehlly, Y., Schimd, C., Trinchieri, G., & Vollmer, B.
Bibliographic Code: 2021A&A...646A.139B
Keywords: galaxies: clusters: general, galaxies: clusters: individual: Virgo, galaxies: evolution, galaxies: interactions, galaxies: ISM, Astrophysics - Astrophysics of Galaxies
Abstract: We study the IB(s)m galaxy IC 3476 observed in the context of the Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE), a blind narrow-band Hα+[NII] imaging survey of the Virgo cluster carried out with MegaCam at the CFHT. The deep narrow-band image reveals a very pertubed ionised gas distribution that is characterised by a prominent banana-shaped structure in the front of the galaxy formed of giant HII regions crossing the stellar disc. Star-forming structures, at ∼8 kpc from the edges of the stellar disc, are also detected in a deep far-ultraviolet ASTROSAT/UVIT image. This particular morphology indicates that the galaxy is undergoing an almost edge-on ram pressure stripping event. The same Hα+[NII] image also shows that the star formation activity is totally quenched in the leading edge of the disc, where the gas has been removed during the interaction with the surrounding medium. The spectral energy distribution fitting analysis of the multi-frequency data indicates that this quenching episode is very recent (∼50 Myr), and roughly corresponds to an increase of the star formation activity by a factor of ∼161% in the inner regions with respect to that expected for secular evolution. The analysis of these data, whose angular resolution enables the study of the induced effects of the perturbation down to the scale of individual HII regions (r<SUB>eq</SUB> ≃ 40 pc), also suggests that the increase of star formation activity is due to the compression of the gas along the stellar disc of the galaxy, which is able to increase its mean electron density and boost the star formation process producing bright HII regions with luminosities up to L(Hα) ≃ 10<SUP>38</SUP> erg s<SUP>-1</SUP>. The combined analysis of the VESTIGE data with deep IFU spectroscopy gathered with MUSE and with high spectral resolution Fabry Perot data also indicates that the hydrodynamic interaction has deeply perturbed the velocity field of the ionised gas component while leaving that of the stellar disc unaffected. The comparison of the data with tuned high-resolution hydrodynamic simulations accounting for the different gas phases (atomic, molecular, ionised) consistently indicates that the perturbing event is very recent (50-150 Myr), once again confirming that ram pressure stripping is a violent phenomenon that is able to perturb the evolution of galaxies in rich environments on short timescales.
Narrow-band imaging data used to prepare Fig. 1 are available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/646/A139">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/646/A139</A>
Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-French-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France and the University of Hawaii. Based on observations made with ESO Tele- scopes at the La Silla Paranal Observatory under programme ID 095.D-0172. Based on observations collected at the Observatoire de Haute Provence (OHP) (France), operated by the CNRS.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2021A&A...646A.139B">2021A&A...646A.139B</a>
Title: UOCS. III. UVIT catalogue of open clusters with machine learning based membership using Gaia EDR3 astrometry
Authors: Jadhav, V. V., Pennock, C. M., Subramaniam, A., Sagar, R., & Nayak, P. K.
Bibliographic Code: 2021MNRAS.tmp..246J
Keywords: (stars:) Hertzsprung, Russell and colour, magnitude diagrams, ultraviolet: stars, (Galaxy:) open clusters and associations: individual: Berkeley 67, King 2, NGC 2420, NGC 2477, NGC 2682, NGC 6940, methods: data analysis, catalogues, Astrophysics - Astrophysics of Galaxies, Astrophysics - Solar and Stellar Astrophysics
Abstract: We present a study of six open clusters (Berkeley 67, King 2, NGC 2420, NGC 2477, NGC 2682 and NGC 6940) using the Ultra Violet Imaging Telescope (UVIT) aboard ASTROSAT and Gaia EDR3. We used combinations of astrometric, photometric and systematic parameters to train and supervise a machine learning algorithm along with a Gaussian mixture model for the determination of cluster membership. This technique is robust, reproducible and versatile in various cluster environments. In this study, the Gaia EDR3 membership catalogues are provided along with classification of the stars as members, candidates and field in the six clusters. We could detect 200-2500 additional members using our method with respect to previous studies, which helped estimate mean space velocities, distances, number of members and core radii. UVIT photometric catalogues, which include blue stragglers, main-sequence and red giants are also provided. From UV-Optical colour-magnitude diagrams, we found that majority of the sources in NGC 2682 and a few in NGC 2420, NGC 2477 and NGC 6940 showed excess UV flux. NGC 2682 images have ten white dwarf detection in far-UV. The far-UV and near-UV images of the massive cluster NGC 2477 have 92 and 576 members respectively, which will be useful to study the UV properties of stars in the extended turn-off and in various evolutionary stages from main-sequence to red clump. Future studies will carry out panchromatic and spectroscopic analysis of noteworthy members detected in this study.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2021MNRAS.tmp..246J">2021MNRAS.tmp..246J</a>
Title: AstroSat soft X-ray observations of the symbiotic recurrent nova V3890 Sgr during its 2019 outburst
Authors: Singh, K. P., Girish, V., Pavana, M., Ness, J.-U., Anupama, G. C., & Orio, M.
Bibliographic Code: 2021MNRAS.501...36S
Keywords: stars: abundances, (stars:) novae, cataclysmic variables, X-rays: stars, novae: individual (V3890 Sgr), Astrophysics - High Energy Astrophysical Phenomena, Astrophysics - Solar and Stellar Astrophysics
Abstract: Two long AstroSat Soft X-ray Telescope observations were taken of the third recorded outburst of the symbiotic recurrent nova V3890 Sgr. The first observing run, 8.1-9.9 d after the outburst, initially showed a stable intensity level with a hard X-ray spectrum that we attribute to shocks between the nova ejecta and the pre-existing stellar companion. On day 8.57, the first, weak, signs appeared of supersoft source (SSS) emission powered by residual burning on the surface of the white dwarf. The SSS emission was observed to be highly variable on time-scales of hours. After day 8.9, the SSS component was more stable and brighter. In the second observing run, on days 15.9-19.6 after the outburst, the SSS component was even brighter but still highly variable. The SSS emission was observed to fade significantly during days 16.8-17.8 followed by re-brightening. Meanwhile, the shock component was stable, leading to increase in hardness ratio during the period of fading. AstroSat and XMM-Newton observations have been used to study the spectral properties of V3890 Sgr to draw quantitative conclusions even if their drawback is model dependent. We used the XSPEC to fit spectral models of plasma emission, and the best fits are consistent with the elemental abundances being lower during the second observing run compared to the first for spectra ≥1 keV. The SSS emission is well fitted by non-local thermal equilibrium model atmosphere used for white dwarfs. The resulting spectral parameters, however, are subject to systematic uncertainties such as completeness of atomic data.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2021MNRAS.501...36S">2021MNRAS.501...36S</a>
Title: UV spectroscopy confirms SU Lyn to be a symbiotic star
Authors: Kumar, V., Srivastava, M. K., Banerjee, D. P. K., & Joshi, V.
Bibliographic Code: 2021MNRAS.500L..12K
Keywords: methods: observational, techniques: spectroscopic, white dwarfs, ultraviolet: general, ultraviolet: stars, Astrophysics - Solar and Stellar Astrophysics
Abstract: SU Lyn, a star that ostensibly appears to be an unremarkable late M type giant, has recently been proposed to be a symbiotic star largely based on its hard X-ray properties. The star does not display, in low-resolution optical spectra, the high excitation lines typically seen in the spectra of symbiotic stars. In this work, ultraviolet (UV), optical, and near-infrared observations are presented, aimed at exploring and strengthening the proposed symbiotic classification for this star. Our far-UV 1300-1800 Å spectrum of SU Lyn, obtained with the ASTROSAT mission's UVIT payload, shows emission lines of Si iv, C iv, O iii, and N iii in a spectrum typical of symbiotic stars. The UV spectrum robustly confirms SU Lyn's symbiotic nature. The detection of high excitation lines in a high-resolution optical spectrum further consolidates its symbiotic nature. As is being recognized, the potential existence of other similar symbiotic systems could significantly impact the census of symbiotic stars in the Galaxy.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2021MNRAS.500L..12K">2021MNRAS.500L..12K</a>
Title: A multiwavelength study of the dual nuclei in Mrk 212
Authors: Rubinur, K., Kharb, P., Das, M., Rahna, P. T., Honey, M., Paswan, A., Vaddi, S., & Murthy, J.
Bibliographic Code: 2021MNRAS.500.3908R
Keywords: galaxies: active, galaxies: formation, radio continuum: galaxies, galaxies: nuclei, galaxies: star formation, Astrophysics - Astrophysics of Galaxies
Abstract: We present radio observations of the galaxy merger remnant Mrk 212 with the Karl G. Jansky Very Large Array (VLA) and the upgraded Giant Meter Radio Telescope (uGMRT). Mrk 212 has two previously known radio sources associated with the two optical nuclei, S1 and S2, with a projected separation of ∼6 kpc, making it a dual active galactic nuclei (AGN) candidate. Our new 15-GHz VLA observations reveal that S1 is a double radio source centred around the optical nucleus; its total extent is ∼750 pc its average 1.4-8.5 GHz spectral index is -0.81 ± 0.06. S1 therefore resembles a compact symmetric object. The 15-GHz VLA image identifies the radio source at S2 to be a compact core. Our radio observations therefore strongly support the presence of a dual AGN in Mrk 212. The optical emission line flux ratios obtained from the Himalayan Chandra Telescope (HCT) observations however, show that S1 and S2 both fall in the AGN + star formation (SF) region of the Baldwin, Philips and Terlevich (BPT) diagram. Weak AGN lying in the SF or AGN + SF intermediate regions in the BPT diagram have indeed been reported in the literature; our sources clearly fall in the same category. We find an extended radio structure in our newly reduced 8.5-GHz-VLA data, that is offset by ∼1 arcsec from the optical nucleus S2. New deep FUV and NUV observations with the Ultraviolet Imaging Telescope aboard AstroSat reveal SF knots around S2 as well as kpc-scale tidal tails; the SF knots around S2 coincide with the extended radio structure detected at 8.5 GHz. The radio spectral indices are consistent with SF. Any possible association with the AGN in S2 is unclear at this stage.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2021MNRAS.500.3908R">2021MNRAS.500.3908R</a>
Title: Multiwavelength study of different flaring and low-activity states of blazar 4C+21.35
Authors: Bhattacharya, D., Mohana A, K., Bhattacharyya, S., Bhatt, N., & Stalin, C. S.
Bibliographic Code: 2021MNRAS.500.1127B
Keywords: galaxies: active, galaxies: jets, gamma-rays: galaxies, X-rays: galaxies-quasars: individual: 4C+21.35
Abstract: Blazars, a class of active galactic nuclei, emit over the entire accessible electromagnetic spectrum and modelling of their broad-band spectral energy distribution (SED) is the key to constrain the underlying emission mechanisms. Here we report the results on the one-zone leptonic emission modelling carried out on the blazar 4C+21.35 using multiwavelength data spanning over the period 2008-2018. Broad-band SED modelling using γ-ray data from Fermi-Large Area Telescope, X-ray data from Swift-XRT and AstroSat, and UV-optical data from Swift-UVOT, AstroSat, and Catalina Real-Time Transient Survey was carried out at seven different epochs, including three γ-ray flaring episodes and four quiescent periods (three long-term averaged ones and one during AstroSat observing period). Our SED modelling suggests that two compact emission regions originating at a different time outside the broad-line region and moving away from the core with variation primarily in the jet electron spectra can explain the emission from the high-, moderate-, and low-activity periods. The emissions from high- and first low-activity states are likely to have originated in the first region. The moderate- and second low-activity states are likely due to the second emission region with fresh particle acceleration/injection at a later time.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2021MNRAS.500.1127B">2021MNRAS.500.1127B</a>
Title: AstroSat observations of the first Galactic ULX pulsar Swift J0243.6+6124
Authors: Beri, A., Naik, S., Singh, K. P., Jaisawal, G. K., Bhattacharyya, S., Charles, P., Ho, W. C. G., Maitra, C., Bhattacharya, D., Dewangan, G. C., Middleton, M., Altamirano, D., Gandhi, P., & Raichur, H.
Bibliographic Code: 2021MNRAS.500..565B
Keywords: pulsars: individual: (Swift J0243.6+6124), X-rays: binaries, Astrophysics - High Energy Astrophysical Phenomena
Abstract: Swift J0243.6+6124, the first Galactic ultraluminous X-ray pulsar, was observed during its 2017-2018 outburst with AstroSat at both sub- and super-Eddington levels of accretion with X-ray luminosities of L<SUB>X</SUB> ∼ 7 × 10<SUP>37</SUP> and 6 × 10<SUP>38</SUP> erg s<SUP>-1</SUP>, respectively. Our broad-band timing and spectral observations show that X-ray pulsations at ${\sim}9.85~\rm {s}$ have been detected up to 150 keV when the source was accreting at the super-Eddington level. The pulse profiles are a strong function of both energy and source luminosity, showing a double-peaked profile with pulse fraction increasing from ∼ $10{{{\ \rm per\ cent}}}$ at $1.65~\rm {keV}$ to 40-80 ${{\ \rm per\ cent}}$ at $70~\rm {keV}$ . The continuum X-ray spectra are well modelled with a high-energy cut-off power law (Γ ∼ 0.6-0.7) and one or two blackbody components with temperatures of ∼0.35 and $1.2~\rm {keV}$ , depending on the accretion level. No iron line emission is observed at sub-Eddington level, while a broad emission feature at around 6.9 keV is observed at the super-Eddington level, along with a blackbody radius ( $121\!-\!142~\rm {km}$ ) that indicates the presence of optically thick outflows.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2021MNRAS.500..565B">2021MNRAS.500..565B</a>
Title: Crust structure and thermal evolution of neutron stars in soft X-ray transients
Authors: Potekhin, A. Y., & Chabrier, G.
Bibliographic Code: 2021A&A...645A.102P
Keywords: stars: neutron, dense matter, X-rays: binaries, X-rays: individuals: MXB 1659-29, Astrophysics - High Energy Astrophysical Phenomena
Abstract: Context. The thermal evolution of neutron stars in soft X-ray transients (SXTs) is sensitive to the equation of state, nucleon superfluidity, and the composition and structure of the crust. Carrying out comparisons of the observations of their crust cooling with simulations offers a powerful tool for verifying theoretical models of dense matter.
Aims: We study the effect of physics input on the thermal evolution of neutron stars in SXTs. In particular, we consider different modern models of the sources of deep crustal heating during accretion episodes and the effects brought on by impurities embedded in the crust during its formation.
Methods: We simulated the thermal structure and evolution of episodically accreting neutron stars under different assumptions regarding the crust composition and on the distribution of heat sources and impurities. For the non-accreted crust, we considered the nuclear charge fluctuations that arise at crust formation. For the accreted crust, we compared different theoretical models of composition and internal heating. We also compared the results of numerical simulations to observations of the crust cooling in SXT MXB 1659-29.
Results: The non-accreted part of the inner crust of a neutron star can have a layered structure, with almost pure crystalline layers interchanged with layers composed of mixtures of different nuclei. The latter layers have relatively low thermal conductivities, which has an effect on the thermal evolution of the transients. The impurity distribution in the crust strongly depends on models of the dense matter and the crust formation scenario. The shallow heating that is needed to reach an agreement between the theory and the observations depends on characteristics of the crust and envelope.
Data are only available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/645/A102">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/645/A102</A>
URL: <a href="https://ui.adsabs.harvard.edu/abs/2021A&A...645A.102P">2021A&A...645A.102P</a>
Title: AstroSat view of GRS 1915+105 during the soft state: detection of HFQPOs and estimation of mass and spin
Authors: Sreehari, H., Nandi, A., Das, S., Agrawal, V. K., Mandal, S., Ramadevi, M. C., & Katoch, T.
Bibliographic Code: 2020MNRAS.499.5891S
Keywords: accretion, accretion discs, black hole physics, X-rays: binaries, Astrophysics - High Energy Astrophysical Phenomena
Abstract: We report the results of AstroSat observations of GRS 1915+105 obtained using 100 ks Guaranteed Time during the soft state. The colour-colour diagram indicates a variability class of δ with the detection of high-frequency quasi-periodic oscillation (HFQPO) in the power density spectra. The HFQPO is seen to vary in the frequency range of 67.96-70.62 Hz with percentage rms ∼0.83-1.90 per cent and significance varying from 1.63 to 7.75. The energy dependent power spectra show that the HFQPO features are dominant only in 6-25 keV energy band. The broad-band energy spectra (0.7-50 keV) of Soft X-ray Telescope and Large Area X-ray Proportional Counter modelled with nthComp and powerlaw imply that the source has an extended corona in addition to a compact 'Comptonizing corona' that produces high-energy emission and exhibits HFQPOs. The broad-band spectral modelling indicates that the source spectra are well described by thermal Comptonization with electron temperature (kT<SUB>e</SUB>) of 2.07-2.43 keV and photon index (Γ<SUB>nth</SUB>) between 1.73 and 2.45 with an additional powerlaw component of photon index (Γ<SUB>PL</SUB>) between 2.94 and 3.28. The norm of nthComp component is high (∼8) during the presence of strong HFQPO and low (∼3) during the absence of HFQPO. Further, we model the energy spectra with the kerrbb model to estimate the accretion rate, mass, and spin of the source. Our findings indicate that the source accretes at super-Eddington rate of $1.17\!-\!1.31~ \dot{M}_{\rm Edd}$ . Moreover, we find the mass and spin of the source as 12.44-13.09 M<SUB>⊙</SUB> and 0.990-0.997 with $90{{\ \rm per\ cent}}$ confidence suggesting that GRS 1915+105 is a maximally rotating stellar mass X-ray binary black hole source.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.499.5891S">2020MNRAS.499.5891S</a>
Title: Coronal vertical structure variations in normal branch of GX 17+2: AstroSat's SXT and LAXPC perspective
Authors: Malu, S., Sriram, K., & Agrawal, V. K.
Bibliographic Code: 2020MNRAS.499.2214M
Keywords: accretion, accretion discs, binaries: close, stars: individual: GX 17+2, X-rays: binaries
Abstract: We performed spectro-temporal analysis in the 0.8-50 keV energy band of the neutron star Z source GX 17+2 using AstroSat Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) data. The source was found to vary in the normal branch (NB) of the hardness-intensity diagram. Cross-correlation studies of LAXPC light curves in soft and hard X-ray band unveiled anticorrelated lags of the order of few hundred seconds. For the first time, cross-correlation studies were performed using SXT soft and LAXPC hard light curves and they exhibited correlated and anticorrelated lags of the order of a hundred seconds. Power density spectrum displayed normal branch oscillations (NBOs) of 6.7-7.8 Hz (quality factor 1.5-4.0). Spectral modelling resulted in inner disc radius of ∼12-16 km with Γ ∼ 2.31-2.44 indicating that disc is close to the innermost stable circular orbit and a similar value of disc radius was noticed based on the reflection model. Different methods were used to constrain the corona size in GX 17+2. Using the detected lags, corona size was found to be 27-46 km (β = 0.1, β = v<SUB>corona</SUB>/v<SUB>disc</SUB>) and 138-231 km (β = 0.5). Assuming the X-ray emission to be arising from the boundary layer (BL), its size was determined to be 57-71 km. Assuming that BL is ionizing the disc's inner region, its size was constrained to ∼19-86 km. Using NBO frequency, the transition shell radius was found to be around 32 km. Observed lags and no movement of the inner disc front strongly indicate that the varying corona structure is causing the X-ray variation in the NB of Z source GX 17+2.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.499.2214M">2020MNRAS.499.2214M</a>
Title: UVIT/AstroSat studies of blue straggler stars and post-mass transfer systems in star clusters: detection of one more blue lurker in M67
Authors: Subramaniam, A., Pandey, S., Jadhav, V. V., & Sahu, S.
Bibliographic Code: 2020JApA...41...45S
Keywords: Blue stragglers, open clusters and associations: individual: M67, stars: rotation, white dwarfs, ultraviolet: stars, Astrophysics - Solar and Stellar Astrophysics
Abstract: The blue straggler stars (BSSs) are main-sequence (MS) stars, which have evaded stellar evolution by acquiring mass while on the MS. The detection of extremely low mass (ELM) white dwarf (WD) companions to two BSSs and one yellow straggler star (YSS) from our earlier study using UVIT/AstroSat, as well as WD companions to main-sequence stars (known as blue lurkers) suggest a good fraction of post-mass transfer binaries in M67. Using deeper UVIT observations, here we report the detection of another blue lurker in M67, with an ELM WD companion. The post-mass transfer systems with the presence of ELM WDs, including BSSs, are formed from Case A/B mass transfer and are unlikely to show any difference in surface abundances. We find a correlation between the temperature of the WD and the v sini of the BSSs. We also find that the progenitors of the massive WDs are likely to belong to the hot and luminous group of BSSs in M67. The only detected BSS+WD system by UVIT in the globular cluster NGC 5466 has a normal WD and suggests that open cluster like environment might be present in the outskirts of low density globular clusters.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020JApA...41...45S">2020JApA...41...45S</a>
Title: The horizontal branch morphology of the globular cluster NGC 1261 using AstroSat
Authors: Rani, S., Pandey, G., Subramaniam, A., Sahu, S., & Rao, N. K.
Bibliographic Code: 2020JApA...41...35R
Keywords: Galaxy: globular clusters: individual: NGC 1261, stars: horizontal-branch, stars: blue stragglers, stars: Hertzsprung-Russell and colour-magnitude diagrams, Astrophysics - Solar and Stellar Astrophysics
Abstract: We present the results obtained from the UV photometry of the globular cluster NGC 1261 using far-UV (FUV) and near-UV (NUV) images acquired with the Ultraviolet Imaging Telescope (UVIT) on-board the AstroSat satellite. We utilized the UVIT data combined with HST, GAIA, and ground-based optical photometric data to construct the different UV colour-magnitude diagrams (CMDs). We detected blue HB (BHB), and two extreme HB (EHB) stars in FUV, whereas full HB, i.e., red HB (RHB), BHB as well as EHB is detected in NUV CMDs. The 2 EHB stars, identified in both NUV and FUV, are confirmed members of the cluster. The HB stars form a tight sequence in UV-optical CMDs, which is almost aligned with Padova isochrones. This study sheds light on the significance of UV imaging to probe the HB morphology in GCs.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020JApA...41...35R">2020JApA...41...35R</a>
Title: Observations with the 3.6-meter Devasthal optical telescope
Authors: Sagar, R., Kumar, B., & Sharma, S.
Bibliographic Code: 2020JApA...41...33S
Keywords: Optical telescope, Sky performance, Detection limits at optical and near-infrared wavelengths, Astrophysics - Instrumentation and Methods for Astrophysics
Abstract: The 3.6-meter Indo-Belgian Devasthal optical telescope (DOT) has been used for optical and near-infrared (NIR) observations of celestial objects. The telescope has detected stars of B =24.5 ±0.2 ,R =24.6 ±0.12 and g =25.2 ±0.2 mag in exposure times of 1200, 4320 and 3600 s respectively. In one hour of exposure time, a distant galaxy of 24.3 ± 0.2 mag and point sources of ∼25 mag have been detected in the SDSS i band. The NIR observations show that stars up to J =20 ±0.1 ,H =18.8 ±0.1 and K =18.2 ±0.1 mag can be detected in effective exposure times of 500, 550 and 1000 s respectively. The nbL band sources brighter than ∼9.2 mag and strong (≥0.4 Jy) PAH emitting sources like Sh 2-61 can also be observed with the 3.6-meter DOT. A binary star with angular separation of 0<SUP>.″</SUP>4 has been resolved by the telescope. Sky images with sub-arcsec angular resolutions are observed with the telescope at wavelengths ranging from optical to NIR for a good fraction of observing time. The on-site performance of the telescope is found to be at par with the performance of other, similar telescopes located elsewhere in the world. Owing to the advantage of its geographical location, the 3.6-meter DOT can provide optical and NIR observations for a number of frontline galactic and extra-galactic astrophysical research problems, including optical follow-up of GMRT and AstroSat sources and optical transient objects.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020JApA...41...33S">2020JApA...41...33S</a>
Title: A spectral study of the black hole X-ray binary MAXI J1820+070 with AstroSat and NuSTAR
Authors: Chakraborty, S., Navale, N., Ratheesh, A., & Bhattacharyya, S.
Bibliographic Code: 2020MNRAS.498.5873C
Keywords: accretion, accretion discs, methods: data analysis, X-rays: binaries, X-rays: individual: MAXI J1820+070, Astrophysics - High Energy Astrophysical Phenomena
Abstract: MAXI J1820+070 is a newly discovered transient black hole X-ray binary, which showed several spectral and temporal features. In this work, we analyse the broad-band X-ray spectra from all three simultaneously observing X-ray instruments onboard AstroSat, as well as contemporaneous X-ray spectra from NuSTAR, observed during the hard state of MAXI J1820+070 in 2018 March. Implementing a combination of multicolour disc model, relativistic blurred reflection model RELXILLLPCP, and a distant reflection in the form of XILLVERCP, we achieve reasonable and consistent fits for AstroSat and NuSTAR spectra. The best-fitting model suggests a low temperature disc (kT<SUB>in</SUB> ∼ 0.3 keV), iron overabundance (A<SUB>Fe</SUB> ∼ 4-5 solar), a short lamp-post corona height (h ≲ 8R<SUB>g</SUB>), and a high corona temperature (kT<SUB>e</SUB> ∼ 115-150 keV). Addition of a second Comptonization component leads to a significantly better fit, with the kT<SUB>e</SUB> of the second Comptonization component being ∼14-18 keV. Our results from independent observations with two different satellites in a similar source state indicate an inhomogeneous corona, with decreasing temperature attributed to increasing height. Besides, utilizing the broader energy coverage of AstroSat, we estimate the black hole mass to be 6.7-13.9 M<SUB>☉</SUB>, consistent with independent measurements reported in the literature.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.498.5873C">2020MNRAS.498.5873C</a>
Title: Revisiting the spectral and timing properties of 4U 1909+07 with NuSTAR and Astrosat
Authors: Jaisawal, G. K., Naik, S., Ho, W. C. G., Kumari, N., Epili, P., & Vasilopoulos, G.
Bibliographic Code: 2020MNRAS.498.4830J
Keywords: stars: neutron, pulsars: individual: 4U 1909+07, X-rays: stars, Astrophysics - High Energy Astrophysical Phenomena, Astrophysics - Solar and Stellar Astrophysics
Abstract: We present the results obtained from the analysis of high-mass X-ray binary pulsar 4U 1909+07 using NuSTAR and Astrosat observations in July 2015 and 2017, respectively. X-ray pulsations at ≍604 s are clearly detected in our study. Based on the long-term spin-frequency evolution, the source is found to spun-up in the last 17 yr. We observed a strongly energy-dependent pulse profile that evolved from a complex broad structure in soft X-rays into a profile with a narrow emission peak followed by a plateau in energy ranges above 20 keV. This behaviour ensured a positive correlation between the energy and pulse fraction. The pulse profile morphology and its energy evolution are almost similar during both the observations, suggesting a persistent emission geometry of the pulsar over time. The broad-band energy spectrum of the pulsar is approximated by an absorbed high-energy exponential cut-off power-law model with iron emission lines. In contrast to the previous report, we found no statistical evidence for the presence of cyclotron absorption features in the X-ray spectra. We performed phase-resolved spectroscopy using data from the NuSTAR observation. Our results showed a clear signature of absorbing material at certain pulse phases of the pulsar. These findings are discussed in terms of stellar wind distribution and its effect on the beam geometry of this wind-fed accreting neutron star. We also reviewed the subsonic quasi-spherical accretion theory and its implication on the magnetic field of 4U 1909+07 depending on the global spin-up rate.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.498.4830J">2020MNRAS.498.4830J</a>
Title: Estimation of the black hole spin in LMC X-1 using AstroSat
Authors: Mudambi, S. P., Rao, A., Gudennavar, S. B., Misra, R., & Bubbly, S. G.
Bibliographic Code: 2020MNRAS.498.4404M
Keywords: accretion, accretion discs, black hole physics, X-rays: binaries, X-rays: individual: stars, Astrophysics - High Energy Astrophysical Phenomena
Abstract: LMC X-1, a persistent, rapidly rotating, extra-galactic, black hole X-ray binary (BHXB) discovered in 1969, has always been observed in its high soft state. Unlike many other BHXBs, the black hole mass, source distance, and binary orbital inclination are well established. In this work, we report the results of simultaneous broad-band spectral studies of LMC X-1 carried out using the data from Soft X-ray Telescope and Large Area X-ray Proportional Counter aboard AstroSat as observed on 2016 November 26 and 2017 August 28. The combined spectrum was modelled with a multicolour blackbody emission (diskbb), a Gaussian along with a Comptonization component (simpl) in the energy range 0.7-30.0 keV. The spectral analysis revealed that the source was in its high soft state (Γ = 2.67 $^{+0.24}_{-0.24}$ and Γ = 2.12 $^{+0.19}_{-0.20}$ ) with a hot disc (kT<SUB>in</SUB> = 0.86 $^{+0.01}_{-0.01}$ and kT<SUB>in</SUB> = 0.87 $^{+0.02}_{-0.02}$ ). Thermal disc emission was fitted with a relativistic model (kerrbb) and spin of the black hole was estimated to be 0.93 $^{+0.01}_{-0.01}$ and 0.93 $^{+0.04}_{-0.03}$ (statistical errors) for the two Epochs through X-ray continuum-fitting, which agrees with the previous results.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.498.4404M">2020MNRAS.498.4404M</a>
Title: Proxima Centauri - the nearest planet host observed simultaneously with AstroSat, Chandra, and HST
Authors: Lalitha, S., Schmitt, J. H. M. M., Singh, K. P., Schneider, P. C., Loyd, R. O. P., France, K., Predehl, P., Burwitz, V., & Robrade, J.
Bibliographic Code: 2020MNRAS.498.3658L
Keywords: stars: activity, stars: coronae, stars: individual: Proxima Centauri, stars: late-type, stars: low-mass, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Earth and Planetary Astrophysics, Astrophysics - High Energy Astrophysical Phenomena
Abstract: Our nearest stellar neighbour, Proxima Centauri, is a low-mass star with spectral type dM5.5 and hosting an Earth-like planet orbiting within its habitable zone. However, the habitability of the planet depends on the high-energy radiation of the chromospheric and coronal activity of the host star. We report the AstroSat, Chandra, and HST observation of Proxima Centauri carried out as part of the multiwavelength simultaneous observational campaign. Using the soft X-ray data, we probe the different activity states of the star. We investigate the coronal temperatures, emission measures and abundance. Finally, we compare our results with earlier observations of Proxima Centauri.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.498.3658L">2020MNRAS.498.3658L</a>
Title: Identifying the radiative components responsible for quasi-periodic oscillations of black hole systems
Authors: Garg, A., Misra, R., & Sen, S.
Bibliographic Code: 2020MNRAS.498.2757G
Keywords: accretion, accretion discs, black hole physics, X-rays: binaries, X-rays: individual: GRS 1915+105, Astrophysics - High Energy Astrophysical Phenomena
Abstract: While the dynamical origin of the variability observed in Galactic black hole systems, such as quasi-periodic oscillations (QPOs), is still a matter of debate, insight into the radiative components responsible for such behaviour can be obtained by studying their energy-dependent temporal behaviour. In particular, one needs to ascertain which variations of the parameters of the best-fitting time-averaged spectral components reproduce the observed energy-dependent fractional rms and time-lags. However, to obtain meaningful interpretation, the standard spectral component parameters have to be recast to physically relevant ones. Then, the energy-dependent temporal variations that their fluctuations will cause, needs to be predicted and compared with observations. In this work, we describe a generic method to do this and apply the technique to the ∼3-4 Hz QPOs observed in the black hole system GRS 1915+105 as observed by AstroSat where the time-averaged spectra can be represented by emission from a truncated disc and hot thermal Comptonizing coronae in the inner regions. We find that the QPOs and their harmonic can be explained in terms of correlated local accretion rate variations in the disc, the truncated disc radius, the optical depth and the heating rate of the coronae with time-delays between them. We highlight the potential of such techniques to unravel the radiative process responsible for variability using high-quality spectral and temporal data from AstroSat and NICER.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.498.2757G">2020MNRAS.498.2757G</a>
Title: AstroSat observation of the Be/X-ray binary Pulsar 3A 0726-260 (4U 0728-25)
Authors: Roy, J., Agrawal, P. C., Singari, B., & Misra, R.
Bibliographic Code: 2020RAA....20..155R
Keywords: Be/X-ray binary, pulsar, 4U 0728-25, Astrophysics - High Energy Astrophysical Phenomena
Abstract: Results on timing and spectral properties of the Be/X-ray binary pulsar 3A 0726-260 (4U 0728-25) are presented. The binary was observed on 2016 May 6-7 with the Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) instruments onboard the AstroSat satellite. During this observation, the source was in non-flaring persistent state at a flux level of ∼ 8.6 ± 0.3 ×10<SUP>-11</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP> in 0.4-20 keV. Strong X-ray pulsations with a period of 103.144±0.001 s are detected in 0.3-7 keV with the SXT and in 3-40 keV with the LAXPC. The pulse profile is energy dependent, and there is an indication that the pulse shape changes from a broad single pulse to a double pulse at higher energy. At energies above 20 keV, we report the first time detection of pulsation period 103.145±0.001 s and the double peaked pulse profile from the source. The energy spectrum of the source is derived from the combined analysis of the SXT and LAXPC spectral data in 0.4-20 keV. The best spectral fit is obtained by a power law model with a photon index (1.7±0.03) with high energy spectral cut-off at 12.9 ± 0.7 keV. A broad iron line at ∼ 6.3 keV is detected in the energy spectrum. We briefly discuss the implications of these results.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020RAA....20..155R">2020RAA....20..155R</a>
Title: AstroSat view of LMC X-2: evolution of broad-band X-ray spectral properties along a complete Z-track
Authors: Agrawal, V. K., & Nandi, A.
Bibliographic Code: 2020MNRAS.497.3726A
Keywords: accretion, accretion discs, X-rays: binaries, X-rays: individual: LMC X-2, Astrophysics - High Energy Astrophysical Phenomena
Abstract: In this paper, we report the first results of the extragalactic Z-source Large Magellanic Cloud (LMC) X-2 obtained using the ∼140 ks observations with Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) onboard AstroSat. The Hardness-Intensity Diagram created with the LAXPC data revealed a complete Z-pattern of the source, showing all the three branches. We studied the evolution of the broad-band X-ray spectra in the energy range of 0.5-20.0 keV along the Z-track, a first such study of this source. The X-ray spectra of the different parts of the Z-pattern were well described by an absorbed Comptonized component. An absence of the accretion disc component suggests that the disc is most probably obscured by a Comptonized region. The best fit electron temperature (kT<SUB>e</SUB>) was found to be in the range of 1.7-2.1 keV and optical depth (τ) was found to be in the range of 13.2-17.5. The optical depth (τ) increased as the source moved from the normal/flaring branch (NB/FB) vertex to the upper part of the FB, suggesting a possible outflow triggered by a strong radiation pressure. The power density spectra (PDS) of HB and NB could be fitted with a pure power law of index α∼1.68 and 0.83, respectively. We also found a weak evidence of quasi-periodic oscillation (2.8σ) in the FB. The intrinsic luminosity of the source varied between (1.03-1.79) × 10<SUP>38</SUP> erg s<SUP>-1</SUP>. We discuss our results by comparing with other Z-sources and the previous observations of LMC X-2.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.497.3726A">2020MNRAS.497.3726A</a>
Title: AstroSat and MAXI view of the black hole binary 4U 1630-472 during 2016 and 2018 outbursts
Authors: Baby, B. E., Agrawal, V. K., Ramadevi, M. C., Katoch, T., Antia, H. M., Mandal, S., & Nandi, A.
Bibliographic Code: 2020MNRAS.497.1197B
Keywords: accretion, accretion discs, black hole physics, stars: individual: 4U 1630-47, Astrophysics - High Energy Astrophysical Phenomena
Abstract: We present an in-depth spectral and timing analysis of the black hole binary 4U 1630-472 during 2016 and 2018 outbursts as observed by AstroSat and MAXI. The extensive coverage of the outbursts with MAXI is used to obtain the hardness intensity diagram (HID). The source follows a 'c'-shaped profile in agreement with earlier findings. Based on the HIDs of previous outbursts, we attempt to track the evolution of the source during a 'super'-outburst and 'mini'-outbursts. We model the broad-band energy spectra (0.7-20.0 keV) of AstroSat observations of both outbursts using phenomenological and physical models. No Keplerian disc signature is observed at the beginning of 2016 outburst. However, the disc appears within a few hours after which it remains prominent with temperature (T<SUB>in</SUB>) ∼ 1.3 keV and increase in photon index (Γ) from 1.8 to 2.0, whereas the source was at a disc dominant state throughout the AstroSat campaign of 2018 outburst. Based on the HIDs and spectral properties, we classify the outbursts into three different states - the 'canonical' hard and soft states along with an intermediate state. Evolution of rms along different states is seen although no quasi-periodic oscillations are detected. We fit the observed spectra using a dynamical accretion model and estimate the accretion parameters. Mass of the black hole is estimated using inner disc radius, bolometric luminosity, and two-component flow model to be 3-9 M<SUB>☉</SUB>. Finally, we discuss the possible implications of our findings.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.497.1197B">2020MNRAS.497.1197B</a>
Title: Time evolution of cyclotron line of Her X-1: a detailed statistical analysis including new AstroSat data
Authors: Bala, S., Bhattacharya, D., Staubert, R., & Maitra, C.
Bibliographic Code: 2020MNRAS.497.1029B
Keywords: stars: neutron, stars: pulsars: individual: Her X-1, X-rays: binaries, Astrophysics - High Energy Astrophysical Phenomena
Abstract: The cyclotron line feature in the X-ray spectrum of the accretion-powered pulsar Her X-1 has been observed and monitored for over three decades. The line energy exhibited a slow secular decline over the period 1995-2014, with a possible (not confirmed) indication of a reversal thereafter. Recent works have shown that the temporal evolution of the line energy may be modelled as a flattening after an earlier decrease until MJD 55400 (±200). In this work, we present the results of AstroSat observations in the context of earlier data and offer a common interpretation through a detailed study of temporal and flux dependence. We find that the variation of the line energy does not support an upward trend but is consistent with the reported flattening after an earlier decrease until MJD $54487^{+515}_{-469}$ .
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.497.1029B">2020MNRAS.497.1029B</a>
Title: AstroSat UVIT Detections of Chandra X-Ray Sources in M31
Authors: Leahy, D. A., & Chen, Y.
Bibliographic Code: 2020ApJS..250...23L
Keywords: Andromeda Galaxy, Ultraviolet sources, X-ray sources, Low-mass x-ray binary stars, Globular star clusters, 39, 1741, 1822, 939, 656, Astrophysics - Astrophysics of Galaxies
Abstract: An ultraviolet (UV) survey of M31 has been carried out during 2017-19 with the Ultra-Violet Imaging Telescope (UVIT) instrument on board the AstroSat Observatory. Here we match the M31 UVIT source catalog with the Chandra source catalog. We find 67 UVIT/Chandra sources detected in a varying number of UV and X-ray bands. The UV and X-ray photometry is analyzed using power-law and blackbody models. The X-ray types include 15 low-mass X-ray binaries (LMXBs) and five active galactic nuclei. Crossmatches with catalogs of stars, clusters, and other source types yield the following: 20 of the UVIT/Chandra sources match with M31 globular clusters, and nine with foreground stars. Three more globular clusters and two more foreground stars are consistent with the UVIT source positions although outside the Chandra match radius of 1″. The UV emission of the UVIT/Chandra sources associated with globular clusters is consistent with emission from blue horizontal branch stars rather than from the X-ray source. The LMXBs in globular clusters are among the most luminous globular clusters in M31. Comparison with stellar evolutionary tracks shows that the UVIT/Chandra sources with high UV blackbody temperatures are consistent with massive (10-30 M<SUB>⊙</SUB>) stars in M31.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020ApJS..250...23L">2020ApJS..250...23L</a>
Title: An underlying clock in the extreme flip-flop state transitions of the black hole transient Swift J1658.2-4242
Authors: Bogensberger, D., Ponti, G., Jin, C., Belloni, T. M., Pan, H., Nandra, K., Russell, T. D., Miller-Jones, J. C. A., Muñoz-Darias, T., Vynatheya, P., & Vincentelli, F.
Bibliographic Code: 2020A&A...641A.101B
Keywords: accretion, accretion disks, black hole physics, X-rays: binaries, time, Astrophysics - High Energy Astrophysical Phenomena
Abstract:
Aims: Flip-flops are top-hat-like X-ray flux variations, which have been observed in some transient accreting black hole binary systems, and feature simultaneous changes in the spectral hardness and the power density spectrum (PDS). They occur at a crucial time in the evolution of these systems, when the accretion disc emission starts to dominate over coronal emission. Flip-flops remain a poorly understood phenomenon, so we aim to thoroughly investigate them in a system featuring several such transitions.
Methods: Within the multitude of observations of <ASTROBJ>Swift J1658.2-4242</ASTROBJ> during its outburst in early 2018, we detected 15 flip-flops, enabling a detailed analysis of their individual properties and the differences between them. We present observations by XMM-Newton, NuSTAR, Astrosat, Swift, Insight-HXMT, INTEGRAL, and ATCA. We analysed their light curves, searched for periodicities, computed their PDSs, and fitted their X-ray spectra, to investigate the source behaviour during flip-flop transitions and how the interval featuring flip-flops differs from the rest of the outburst.
Results: The flip-flops of Swift J1658.2-4242 are of an extreme variety, exhibiting flux differences of up to 77% within ˜100 s, which is much larger than what has been seen previously. We observed radical changes in the PDS simultaneous with the sharp flux variations, featuring transitions between the quasi-periodic oscillation types C and A, which have never been observed before. Changes in the PDS are delayed, but more rapid than changes in the light curve. Flip-flops occur in two intervals within the outburst, separated by about two weeks in which these phenomena were not seen. Transitions between the two flip-flop states occurred at random integer multiples of a fundamental period of 2.761 ks in the first interval and 2.61 ks in the second. Spectral analysis reveals the high and low flux flip-flop states to be very similar, but distinct from intervals lacking flip-flops. A change of the inner temperature of the accretion disc is responsible for most of the flux difference in the flip-flops. We also highlight the importance of correcting for the influence of the dust scattering halo on the X-ray spectra.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020A&A...641A.101B">2020A&A...641A.101B</a>
Title: Study of recent outburst in the Be/X-ray binary RX J0209.6-7427 with AstroSat: a new ultraluminous X-ray pulsar in the Magellanic Bridge?
Authors: Chandra, A. D., Roy, J., Agrawal, P. C., & Choudhury, M.
Bibliographic Code: 2020MNRAS.495.2664C
Keywords: stars: emission-line, Be, stars: neutron, pulsars: individual: RX J0209.6-7427, galaxies: individual: SMC, X-rays: binaries, X-rays: bursts, Astrophysics - High Energy Astrophysical Phenomena
Abstract: We present the timing and spectral studies of RX J0209.6-7427 during its rare 2019 outburst using observations with the Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) instruments on the AstroSat satellite. Pulsations having a periodicity of 9.29 s were detected for the first time by the NICER mission in the 0.2-10 keV energy band and, as reported here, by AstroSat over a broad energy band covering 0.3-80 keV. The pulsar exhibits a rapid spin-up during the outburst. Energy resolved folded pulse profiles are generated in several energy bands in 3-80 keV. To the best of our knowledge this is the first report of the timing and spectral characteristics of this Be binary pulsar in hard X-rays. There is suggestion of evolution of the pulse profile with energy. The energy spectrum of the pulsar is determined and from the best-fitting spectral values, the X-ray luminosity of RX J0209.6-7427 is inferred to be 1.6 × 10<SUP>39</SUP> erg s<SUP>-1</SUP>. Our timing and spectral studies suggest that this source has features of an ultraluminous X-ray pulsar in the Magellanic Bridge. Details of the results are presented and discussed in terms of the current ideas.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.495.2664C">2020MNRAS.495.2664C</a>
Title: AstroSat Observations of GRO J2058+42 during the 2019 Outburst
Authors: Mukerjee, K., Antia, H. M., & Katoch, T.
Bibliographic Code: 2020ApJ...897...73M
Keywords: 1822, 733, 1108, 142, 153, Astrophysics - High Energy Astrophysical Phenomena
Abstract: We present results from AstroSat observations of the recent outburst of GRO J2058+42, an X-ray pulsar in a Be-binary system. The source was observed by the LAXPC and SXT instruments on AstroSat on 2019 April 10 during the declining phase of its latest giant outburst. Light curves showed a strong pulsation of the pulsar with a period of 194.2201 ± 0.0016 s and a spin-up rate of (1.65 ± 0.06) × 10<SUP>-11</SUP> Hz s<SUP>-1</SUP>. Intermittent flaring was detected in light curves between the 3 and 80 keV energy bands, with an increase in intensity of up to 1.8 times its average intensity. Pulse profiles obtained between the 3 and 80 keV energy bands of the pulsar showed strong dependence on energy. During AstroSat observations, a broad peak was consistently observed in the power density spectrum of the source with a peak oscillation frequency of 0.090 Hz along with its higher harmonics, which may be due to quasi-periodic oscillations, a commonly observed phenomenon in transient X-ray pulsars during their outburst. AstroSat observations also detected cyclotron absorption features in its spectrum corresponding to (9.7-14.4) keV, (19.3-23.8) keV, and (37.8-43.1) keV. The pulse-phase-resolved spectroscopy of the source showed a phase-dependent variation in its energy and the relative strength of these features. The spectrum was well fitted with an absorbed blackbody, Fermi-Dirac cutoff model and alternatively with an absorbed CompTT model. Both of these models were combined with an Fe line and three Gaussian absorption lines to account for the observed cyclotron resonance scattering features in the spectrum.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020ApJ...897...73M">2020ApJ...897...73M</a>
Title: Blazar Variability: A Study of Nonstationarity and the Flux-Rms Relation
Authors: Bhattacharyya, S., Ghosh, R., Chatterjee, R., & Das, N.
Bibliographic Code: 2020ApJ...897...25B
Keywords: 1810, 164, 2035, Astrophysics - High Energy Astrophysical Phenomena, Astrophysics - Astrophysics of Galaxies
Abstract: We analyze X-ray light curves of the blazars Mrk 421, PKS 2155-304, and 3C 273 using observations by the Soft X-ray Telescope on board AstroSat and archival XMM-Newton data. We use light curves of length 30-90 ks from three to four epochs for all three blazars. We apply the autoregressive integrated moving average model, which indicates the variability is consistent with short memory processes for most of the epochs. We show that the power spectral density (PSDs) of the X-ray variabilities of the individual blazars are consistent within uncertainties across the epochs. This implies that the construction of broadband PSD using light curves from different epochs is accurate. However, using certain properties of the variance of the light curves and its segments, we show that the blazars exhibit hints of nonstationarity beyond that due to their characteristic red-noise nature in some of those observations. We find a linear relationship between the root-mean-squared amplitude of variability at shorter timescales and the mean flux level at longer timescales for light curves of Mrk 421 across epochs separated by decades as well as light curves spanning 5 days and ∼10 yr. The presence of a flux-rms relation over very different timescales may imply that, similar to the X-ray binaries and Seyfert galaxies, longer and shorter timescale variabilities are connected in blazars.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020ApJ...897...25B">2020ApJ...897...25B</a>
Title: Detection of thermonuclear X-ray bursts and dips from the X-ray binary 4U 1323-62 with AstroSat/LAXPC
Authors: Bhulla, Y., Roy, J., & Jaaffrey, S. N. A.
Bibliographic Code: 2020RAA....20...98B
Keywords: accretion disc, individual: 4U 1323-62, stars: neutron star, X-rays: binaries, X-rays: thermonuclear bursts, 14, Astrophysics - High Energy Astrophysical Phenomena
Abstract: Using data from the Large Area X-ray Proportional Counter (LAXPC) on the AstroSat satellite, we observed Type-1 thermonuclear X-ray bursts from the low mass X-ray binary neutron star 4U 1323-62. Observations of 4U 1323-62 that were carried out during the performance verification phase of the AstroSat satellite indicated six thermonuclear X-ray bursts in a total effective exposure of ∼49.5 ks for about two consecutive days. Recurrence time of the detected thermonuclear bursts is in accordance with the orbital period of the source, ∼9400 s. Moreover, the light curve of 4U 1323-62 revealed the presence of two dips. We present the results from time-resolved spectroscopy performed during all of the six X-ray bursts and also report the detection of a known low frequency quasi-periodic oscillation (LFQPO) at ∼1 Hz from the source. However, any evidence of kilohertz QPO was not found. We have shown the burst profile at different energy ranges. Assuming a distance of 10 kpc, we observed a mean flux of ∼1.8×10<SUP>-9</SUP> erg cm<SUP>2</SUP> s<SUP>-1</SUP>. The radius of the blackbody is found to be highly consistent with the blackbody temperature and the blackbody flux of the bursts.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020RAA....20...98B">2020RAA....20...98B</a>
Title: Isolation, nucleotide sequencing and genomic comparison of a Novel SXT/R391 ICE mobile genetic element isolated from a municipal wastewater environment
Authors: Slattery, S., Tony Pembroke, J., Murnane, J. G., & Ryan, M. P.
Bibliographic Code: 2020NatSR..10.8716S
Keywords:
Abstract: Integrative Conjugative Elements (ICE's) of the SXT/R391 family have largely been detected in clinical or environmental isolates of Gammaproteobacteria, particularly Vibrio and Proteus species. As wastewater treatment plants accumulate a large and diverse number of such species, we examined raw water samples taken from a municipal wastewater treatment plant initially using SXT/R391 family integrase gene-specific PCR probes to detect the presence of such elements in a directed approach. A positive amplification occurred over a full year period and a subsequent Restriction Fragment Length Polymorphism (RFLP) analysis revealed a very limited diversity in the treatment plant examined. Samples demonstrating positive amplification were cultured using Vibrio and Proteus selective media and PCR amplification tracking was utilized to monitor SXT/R391-ICE family containing strains. This screening procedure resulted in the isolation and identification of a Proteus mirabilis strain harbouring an ICE. Whole-genome sequencing of this ICE containing strain using Illumina sequencing technology revealed a novel 81 kb element that contained 75 open reading frames on annotation but contained no antibiotic or metal resistance determinants. Comparative genomics revealed the element contained a conserved ICE core with one of the insertions containing a novel bacteriophage defence mechanism. This directed isolation suggests that ICE elements are present in the environment without apparent selective pressure but may contain adaptive functions allowing survival in particular environments such as municipal wastewater which are reservoirs for large bacterial phage populations.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020NatSR..10.8716S">2020NatSR..10.8716S</a>
Title: Spectropolarimetric analysis of prompt emission of GRB 160325A: jet with evolving environment of internal shocks
Authors: Sharma, V., Iyyani, S., Bhattacharya, D., Chattopadhyay, T., Vadawale, S. V., & Bhalerao, V. B.
Bibliographic Code: 2020MNRAS.493.5218S
Keywords: polarization, radiation mechanisms: non-thermal, radiation mechanisms: thermal, gamma-ray burst: individual (GRB 160325A), Astrophysics - High Energy Astrophysical Phenomena
Abstract: GRB 160325A is the only bright burst detected by AstroSat CZT Imager in its primary field of view to date. In this work, we present the spectral and polarimetric analysis of the prompt emission of the burst using AstroSat, Fermi, and Niel Gehrels Swift observations. The prompt emission consists of two distinct emission episodes separated by a few seconds of quiescent/ mild activity period. The first emission episode shows a thermal component as well as a low polarization fraction of PF < 37 per cent at 1.5 σ confidence level. On the other hand, the second emission episode shows a non-thermal spectrum and is found to be highly polarized with PF > 43 per cent at 1.5σ confidence level. We also study the afterglow properties of the jet using Swift/XRT data. The observed jet break suggests that the jet is pointed towards the observer and has an opening angle of 1.2° for an assumed redshift, z = 2. With composite modelling of polarization, spectrum of the prompt emission, and the afterglow, we infer that the first episode of emission originates from the photosphere with localized dissipation happening below it, and the second from the optically thin region above the photosphere. The photospheric emission is generated mainly by inverse Compton scattering, whereas the emission in the optically thin region is produced by the synchrotron process. The low radiation efficiency of the burst suggests that the outflow remains baryonic dominated throughout the burst duration with only a subdominant Poynting flux component, and the kinetic energy of the jet is likely dissipated via internal shocks which evolves from an optically thick to optically thin environment within the jet.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.493.5218S">2020MNRAS.493.5218S</a>
Title: Possible detection of a new cyclotron feature in 4U 1700-37
Authors: Bala, S., Roy, J., & Bhattacharya, D.
Bibliographic Code: 2020MNRAS.493.3045B
Keywords: magnetic field, binaries: eclipsing, stars: individual: 4U 1700-37, Astrophysics - High Energy Astrophysical Phenomena
Abstract: We present a spectral and timing study of the high-mass X-ray binary transient source 4U 1700-37 using NuSTAR and ASTROSAT/LAXPC. The source is observed in two different flux states. A combined spectral analysis of NuSTAR's focal plane modules A and B shows the possible hint of a cyclotron line feature at ˜16 keV. The line feature is consistently present in different continuum models with at least 3σ confidence level. We do not detect the presence of a previously reported 39 keV cyclotron line in the combined spectra. A ˜16 keV cyclotron feature would suggest that the compact object is a neutron star with a magnetic field strength ˜2.1 × 10<SUP>12</SUP> G in the emission region. We also find the presence of a rare Ni Kα emission line around 7.6 keV in the NuSTAR spectrum. We searched the NuSTAR and ASTROSAT data for coherent or quasi-periodic oscillation signals but found no evidence in the frequency range 0.1 mHz to 10<SUP>3</SUP> Hz.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.493.3045B">2020MNRAS.493.3045B</a>
Title: AstroSat UVIT Survey of M31: Point-source Catalog
Authors: Leahy, D. A., Postma, J., Chen, Y., & Buick, M.
Bibliographic Code: 2020ApJS..247...47L
Keywords: 1736, 1741, 39
Abstract: An ultraviolet survey of M31 has been carried out during 2017-19 with the UltraViolet Imaging Telescope (UVIT) instrument on board the AstroSat Observatory. Here we analyze far- and near-ultraviolet (FUV and NUV) observations from the M31 UVIT survey, which covers a sky area of ≃3° × 1° with spatial resolution of ≃1″. The observations included six filter bands in the wavelength range of 120-280 nm. The limiting magnitude (AB) in the FUV band (CaF2 filter), which has the largest number of detected sources, is ∼23. The primary product of this work is the M31 UVIT point-source catalog containing positions and photometry. In total ≃75,000 sources were detected at FUV or NUV wavelengths.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020ApJS..247...47L">2020ApJS..247...47L</a>
Title: Additional Calibration of the Ultraviolet Imaging Telescope on Board AstroSat
Authors: Tandon, S. N., Postma, J., Joseph, P., Devaraj, A., Subramaniam, A., Barve, I. V., George, K., Ghosh, S. K., Girish, V., Hutchings, J. B., Kamath, P. U., Kathiravan, S., Kumar, A., Lancelot, J. P., Leahy, D., Mahesh, P. K., Mohan, R., Nagabhushana, S., Pati, A. K., Rao, N. K., Sankarasubramanian, K., Sriram, S., & Stalin, C. S.
Bibliographic Code: 2020AJ....159..158T
Keywords: 1736, 1743, 799, 1740, 1558, Astrophysics - Instrumentation and Methods for Astrophysics
Abstract: Results of the initial calibration of the Ultra-Violet Imaging Telescope (UVIT) were reported earlier by Tandon et al. The results reported earlier were based on the ground calibration as well as the first observations in orbit. Some additional data from the ground calibration and data from more in-orbit observations have been used to improve the results. In particular, extensive new data from in-orbit observations have been used to obtain (a) new photometric calibration that includes (i) zero points, (ii) flat fields, and (iii) saturation; (b) sensitivity variations; (c) spectral calibration for the near ultraviolet (NUV; 2000-3000 Å) and far ultraviolet (FUV; 1300-1800 Å) gratings; (d) point-spread function; and (e) astrometric calibration including distortion. Data acquired over the last three years show continued good performance of UVIT with no reduction in sensitivity in both UV channels.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020AJ....159..158T">2020AJ....159..158T</a>
Title: A broad-band look of the accreting millisecond X-ray pulsar SAX J1748.9-2021 using AstroSat and XMM-Newton
Authors: Sharma, R., Beri, A., Sanna, A., & Dutta, A.
Bibliographic Code: 2020MNRAS.492.4361S
Keywords: accretion, accretion discs, stars: neutron, X-rays: binaries, X-rays: bursts, X-rays: individual (SAX J1748.9-2021), Astrophysics - High Energy Astrophysical Phenomena
Abstract: SAX J1748.9-2021 is a transient accretion powered millisecond X-ray pulsar located in the globular cluster NGC 6440. We report on the spectral and timing analysis of SAX J1748.9-2021 performed on AstroSat data taken during its faint and short outburst of 2017. We derived the best-fitting orbital solution for the 2017 outburst and obtained an average local spin frequency of 442.361098(3) Hz. The pulse profile obtained from 3 to 7 and 7 to 20 keV energy bands suggest constant fractional amplitude ˜0.5 per cent for fundamental component, contrary to previously observed energy pulse profile dependence. Our AstroSat observations revealed the source to be in a hard spectral state. The 1-50 keV spectrum from SXT (Soft X-ray Telescope) and LAXPC (Large Area X-ray Proportional Counter) on-board AstroSat can be well described with a single temperature blackbody and thermal Comptonization. Moreover, we found that the combined spectra from XMM-Newton (EPIC-PN) and AstroSat (SXT + LAXPC) indicated the presence of reflection features in the form of iron (Fe Kα) line that we modelled with the reflection model xillvercp. One of the two X-ray burst observed during the AstroSat/LAXPC observation showed hard X-ray emission (>30 keV) due to Compton up-scattering of thermal photons by the hot corona. Time-resolved analysis performed on the bursts revealed complex evolution in emission radius of blackbody for second burst suggestive of mild photospheric radius expansion.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.492.4361S">2020MNRAS.492.4361S</a>
Title: Correction to: AstroSat/LAXPC view of GX 17+2: spectral evolution along the Z-track
Authors: Agrawal, V. K., Nandi, A., & Ramadevi, M. C.
Bibliographic Code: 2020Ap&SS.365...56A
Keywords:
Abstract: In the Discussion section of the initial online publication an incorrect percentage was specified for the observed contribution of the hard X-ray tail in the middle and upper flaring branch to the total flux of 3.0-80.0 keV. The original article has been corrected.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020Ap&SS.365...56A">2020Ap&SS.365...56A</a>
Title: Unravelling the unusually curved X-ray spectrum of RGB J0710 + 591 using AstroSat observations
Authors: Goswami, P., Sinha, A., Chandra, S., Misra, R., Chitnis, V., Gogoi, R., Sahayanathan, S., Stalin, C. S., Singh, K. P., & Yadav, J. S.
Bibliographic Code: 2020MNRAS.492..796G
Keywords: galaxies: active, BL Lacertae objects: individual: RGB J0710 + 591, X-rays: galaxies, Astrophysics - High Energy Astrophysical Phenomena
Abstract: We report the analysis of simultaneous multiwavelength data of the high-energy-peaked blazar RGB J0710 + 591 from the Large Area X-ray Proportional Counters, Soft X-ray focusing Telescope, and Ultraviolet Imaging Telescope (UVIT) instruments onboard AstroSat. The wide band X-ray spectrum (0.35-30 keV) is modelled as synchrotron emission from a non-thermal distribution of high-energy electrons. The spectrum is unusually curved, with a curvature parameter β<SUB>p</SUB> ∼ 6.4 for a log parabola particle distribution, or a high-energy spectral index p<SUB>2</SUB> > 4.5 for a broken power-law distribution. The spectrum shows more curvature than an earlier quasi-simultaneous analysis of Swift-XRT/NuSTAR data where the parameters were β<SUB>p</SUB> ∼ 2.2 or p<SUB>2</SUB> ∼ 4. It has long been known that a power-law electron distribution can be produced from a region where particles are accelerated under Fermi process and the radiative losses in acceleration site decide the maximum attainable Lorentz factor, γ<SUB>max</SUB>. Consequently, this quantity decides the energy at which the spectrum curves steeply. We show that such a distribution provides a more natural explanation for the AstroSat data as well as the earlier XRT/NuSTAR observation, making this as the first well-constrained determination of the photon energy corresponding to γ<SUB>max</SUB>. This in turn provides an estimate of the acceleration time-scale as a function of magnetic field and Doppler factor. The UVIT observations are consistent with earlier optical/UV measurements and reconfirm that they plausibly correspond to a different radiative component than the one responsible for the X-ray emission.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.492..796G">2020MNRAS.492..796G</a>
Title: Identification of QPO Frequency of GRS 1915+105 as the Relativistic Dynamic Frequency of a Truncated Accretion Disk
Authors: Misra, R., Rawat, D., Yadav, J. S., & Jain, P.
Bibliographic Code: 2020ApJ...889L..36M
Keywords: 14, 1611, 1406, 98, 939, 159, 886, 1388, 5, 1393, 1810, Astrophysics - High Energy Astrophysical Phenomena
Abstract: We have analyzed AstroSat observations of the galactic microquasar system GRS 1915+105, when the system exhibited C-type quasi-periodic oscillations (QPOs) in the frequency range of 3.4-5.4 Hz. The broadband spectra (1-50 keV) obtained simultaneously from the Large Area X-ray Proportional Counter and Soft X-ray Telescope can be well described by a dominant relativistic truncated accretion disk along with thermal Comptonization and reflection. We find that while the QPO frequency depends on the inner radii with a large scatter, a much tighter correlation is obtained when both the inner radii and accretion rate of the disk are taken into account. In fact, the frequency varies just as the dynamic frequency (I.e., the inverse of the sound crossing time) does as predicted decades ago by the relativistic standard accretion disk theory for a black hole with a spin parameter of ∼0.9. We show that this identification has been possible due to the simultaneous broadband spectral coverage with temporal information as obtained from AstroSat.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020ApJ...889L..36M">2020ApJ...889L..36M</a>
Title: AstroSat UVIT Observations of Her X-1
Authors: Leahy, D. A., Postma, J., & Chen, Y.
Bibliographic Code: 2020ApJ...889..131L
Keywords: 1108, 1811
Abstract: An observation of the X-ray binary system Her X-1/HZ Her by the AstroSat UltraViolet Imaging Telescope (UVIT) was carried out in 2018. The observation was taken with the far ultra-violet (FUV) camera of UVIT with the CaF<SUB>2</SUB> filter (125-175 nm band) and lasted ≃0.6 of one binary orbit. Her X-1 was in the late main high state at 35 day phase ≃0.20. Clear orbital modulation of the UVIT light curve is seen, showing egress from eclipse of the neutron star and double-peak shape half an orbital period later. The FUV emission from Her X-1 is shown to arise partly from the accretion disk and partly from the X-ray heated face of Hz Her. We carry out modeling of the system using the Shape code and fit the FUV light curve. New constraints are obtained for the geometric parameters of the tilted and twisted accretion disk around the neutron star, and improvements to the basic disk model are suggested.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020ApJ...889..131L">2020ApJ...889..131L</a>
Title: AstroSat/LAXPC view of GX 17+2: spectral evolution along the Z-track
Authors: Agrawal, V. K., Nandi, A., & Ramadevi, M. C.
Bibliographic Code: 2020Ap&SS.365...41A
Keywords: Accretion, Accretion discs, X-rays: binaries, X-rays: individual, GX 17+2, Astrophysics - High Energy Astrophysical Phenomena
Abstract: In this paper, we present the first results obtained using ∼50 ks observations of the bright low-mass X-ray binary (LMXB) GX 17+2 with Large Area X-ray Proportional Counter (LAXPC) onboard AstroSat. The source traced out a complete Z-track in the hardness intensity diagram (HID). The spectra at different sections of the Z-diagram are well described by either a combination of a thermal Comptonization component, a power-law and a relativistic iron line or a model consisting of a thermal disk component, a single temperature blackbody, a power-law and a relativistic iron line. Fitting the spectra with both phenomenological models suggests that the power-law component is strong in the horizontal branch (HB), becomes weaker as the source moves down the normal branch (NB) and then again becomes stronger as the sources moves up the flaring branch (FB). However, we find that the strength of the power-law component is model dependent, although the trend in the variation of the power-law strength along the Z-track is similar. A simple model composed by a Comptonized emission and power-law component, convolved with the ionized reflection, also describes the spectra very well.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020Ap&SS.365...41A">2020Ap&SS.365...41A</a>
Title: AstroSat detection of Lyman continuum emission from a z = 1.42 galaxy
Authors: Saha, K., Tandon, S. N., Simmonds, C., Verhamme, A., Paswan, A., Schaerer, D., Rutkowski, M., Borgohain, A., Elmegreen, B., Inoue, A. K., Combes, F., Elmegreen, D., & Paalvast, M.
Bibliographic Code: 2020NatAs...4.1185S
Keywords: Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics
Abstract: One of the outstanding problems of current observational cosmology is to understand the nature of sources that produced the bulk of the ionizing radiation after the Cosmic Dark Age. Direct detection of these reionization sources<SUP>1</SUP> is practically infeasible at high redshift (z) due to the steep decline of intergalactic medium transmission<SUP>2,3</SUP>. However, a number of low-z analogues emitting Lyman continuum at 900 å restframe are now detected at z < 0.4 (refs. <SUP>4-8</SUP>) and there are also detections in the range 2.5 < z < 3.5 (refs. <SUP>9-14</SUP>). Here we report the detection of Lyman continuum emission with a high escape fraction (>20%) from a low-mass clumpy galaxy at z = 1.42, in the middle of the redshift range where no detection has been made before and near the peak of the cosmic star-formation history<SUP>15</SUP>. The observation was made in the Hubble Extreme Deep Field<SUP>16</SUP> by the wide-field Ultraviolet Imaging Telescope<SUP>17</SUP> onboard AstroSat<SUP>18</SUP>. This detection of extreme ultraviolet radiation from a distant galaxy at a restframe wavelength of 600 å opens up a new window to constrain the shape of the ionization spectrum. Further observations with AstroSat should substantially increase the sample of Lyman-continuum-leaking galaxies at cosmic noon.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020NatAs...4.1185S">2020NatAs...4.1185S</a>
Title: Unveiling the Temporal Properties of MAXI J1820+070 through AstroSat Observations
Authors: Mudambi, S. P., Maqbool, B., Misra, R., Hebbar, S., Yadav, J. S., Gudennavar, S. B., & S. G., B.
Bibliographic Code: 2020ApJ...889L..17M
Keywords: 159, 1579, 939, 1852, 1578, 1810, 1611, 162, Astrophysics - High Energy Astrophysical Phenomena
Abstract: We present here the results of the first broadband simultaneous spectral and temporal studies of the newly detected black hole binary MAXI J1820+070 as seen by Soft X-ray Telescope and Large Area X-ray Proportional Counter (LAXPC) on board AstroSat. The observed combined spectra in the energy range 0.7-80 keV were well modeled using disk blackbody emission, thermal Comptonization, and a reflection component. The spectral analysis revealed that the source was in its hard spectral state (Γ = 1.61) with a cool disk (kT<SUB>in</SUB> = 0.22 keV). We report the energy dependent time-lag and root mean squared (rms) variability at different frequencies in the energy range 3-80 keV using LAXPC data. We also modeled the flux variability using a single-zone stochastic propagation model to quantify the observed energy dependence of time lag and fractional rms variability, and then compared the results with that of Cygnus X-1. Additionally, we confirm the detection of a quasi-periodic oscillation with the centroid frequency at 47.7 mHz.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020ApJ...889L..17M">2020ApJ...889L..17M</a>
Title: Prompt X-Ray Emission from Fast Radio Bursts—Upper Limits with AstroSat
Authors: Anumarlapudi, A., Bhalerao, V., Tendulkar, S. P., & Balasubramanian, A.
Bibliographic Code: 2020ApJ...888...40A
Keywords: Radio Bursts, Radio transient sources, X-ray bursts, 1339, 2008, 1814, Astrophysics - High Energy Astrophysical Phenomena
Abstract: Fast radio bursts (FRBs) are short-lived (∼ms), energetic transients (having a peak flux density of ∼Jy) with no known prompt emission in other energy bands. We present results of a search for prompt X-ray emissions from 41 FRBs using the Cadmium Zinc Telluride Imager on AstroSat, which continuously monitors ∼70% of the sky. Our searches on various timescales in the 20-200 keV range, did not yield any counterparts in this hard X-ray band. We calculate upper limits on hard X-ray flux, in the same energy range and convert them to upper bounds for η: the ratio of X-ray to radio fluence of FRBs. We find η ≤ 10<SUP>8-10</SUP> for hard X-ray emission. Our results will help constrain the theoretical models of FRBs as the models become more quantitative and nearer, brighter FRBs are discovered.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2020ApJ...888...40A">2020ApJ...888...40A</a>
Title: Solar Cycle Variation of Coronal Temperature, Emission Measure, and Soft X-Ray Irradiance Observed with Yohkoh Soft X-Ray Telescope
Authors: Takeda, A., Acton, L., & Albanese, N.
Bibliographic Code: 2019ApJ...887..225T
Keywords: Solar corona, Solar spectral irradiance, Solar X-ray emission, 1483, 1501, 1536
Abstract: This paper presents the solar soft X-ray irradiance (0.3-3.0 nm) obtained from the mission-long full-Sun X-ray images observed with the Soft X-ray Telescope (SXT) of the Yohkoh mission (1991-2001). The irradiance was calculated using filter-ratio temperatures and emission measures (EMs) from the full-Sun integrated X-ray signals employing a coronal spectrum synthesized with CHIANTI atomic database version 8.0. Dependence of the results on the assumption of elemental abundances is investigated. The SXT irradiance for a shorter wavelength range was also calculated and compared with the low energy flux (0.1-0.8 nm) of the X-Ray Sensor on board the GOES satellite. To incorporate the spatial information of the observed corona, we tried the alternative method to derive irradiance by applying the filter-ratio method to the images of northern hemispheric corona instead of spatially integrated signals from there. The EM weighted average of the spatially resolved temperatures turned out to be generally 20%-30% lower than those derived from the integrated signals, while the total of spatially resolved EMs are accordingly 1.5-2 times higher. This trend is enhanced when solar activity is the lowest in early 1996. The irradiance obtained from the alternative method does not significantly vary from the result from the integrated signals. This provides some validation for the simpler full-Sun integrated method. The temporal variation of the EMs of cool (<1.5 MK), medium, and hot (>2.5 MK) components indicates that the ratio of the hot component relative to the medium component is higher in the ascending phase (mid 1998-2001) than in the descending phase (1992-1995) of the solar activity cycle.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019ApJ...887..225T">2019ApJ...887..225T</a>
Title: GASP XXIII: A Jellyfish Galaxy as an Astrophysical Laboratory of the Baryonic Cycle
Authors: Poggianti, B. M., Ignesti, A., Gitti, M., Wolter, A., Brighenti, F., Biviano, A., George, K., Vulcani, B., Gullieuszik, M., Moretti, A., Paladino, R., Bettoni, D., Franchetto, A., Jaffé, Y. L., Radovich, M., Roediger, E., Tomičić, N., Tonnesen, S., Bellhouse, C., Fritz, J., & Omizzolo, A.
Bibliographic Code: 2019ApJ...887..155P
Keywords: Galaxy evolution, Galaxy clusters, Galaxy processes, 594, 584, 614, Astrophysics - Astrophysics of Galaxies
Abstract: With MUSE, Chandra, VLA, ALMA, and UVIT data from the GASP program, we study the multiphase baryonic components in a jellyfish galaxy (JW100) with a stellar mass 3.2 × 10<SUP>11</SUP> M <SUB>⊙</SUB> hosting an active galactic nucleus (AGN). We present its spectacular extraplanar tails of ionized and molecular gas, UV stellar light, and X-ray and radio continuum emission. This galaxy represents an excellent laboratory to study the interplay between different gas phases and star formation and the influence of gas stripping, gas heating, and AGNs. We analyze the physical origin of the emission at different wavelengths in the tail, in particular in situ star formation (related to Hα, CO, and UV emission), synchrotron emission from relativistic electrons (producing the radio continuum), and heating of the stripped interstellar medium (ISM; responsible for the X-ray emission). We show the similarities and differences of the spatial distributions of ionized gas, molecular gas, and UV light and argue that the mismatch on small scales (1 kpc) is due to different stages of the star formation process. We present the relation Hα-X-ray surface brightness, which is steeper for star-forming regions than for diffuse ionized gas regions with a high [O I]/Hα ratio. We propose that ISM heating due to interaction with the intracluster medium (either for mixing, thermal conduction, or shocks) is responsible for the X-ray tail, observed [O I] excess, and lack of star formation in the northern part of the tail. We also report the tentative discovery in the tail of the most distant (and among the brightest) currently known ULX, a pointlike ultraluminous X-ray source commonly originating in a binary stellar system powered by either an intermediate-mass black hole or a magnetized neutron star.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019ApJ...887..155P">2019ApJ...887..155P</a>
Title: Spectral and Timing Properties of the Galactic X-Ray Transient Swift J1658.2-4242 Using Astrosat Observations
Authors: Jithesh, V., Maqbool, B., Misra, R., T, A. R., Mall, G., & James, M.
Bibliographic Code: 2019ApJ...887..101J
Keywords: Stellar accretion disks, Black hole physics, X-ray transient sources, X-ray sources, X-ray binary stars, Accretion, 1579, 159, 1852, 1822, 1811, 14, Astrophysics - High Energy Astrophysical Phenomena
Abstract: We present the X-ray timing and spectral analysis of the new Galactic X-ray transient Swift J1658.2-4242 observed with the Large Area X-ray Proportional Counter and Soft X-ray Telescope instruments on board Astrosat. We detect prominent C-type quasi-periodic oscillations (QPOs) of frequencies varying from ˜1.5 to ˜6.6 Hz along with distinct second harmonics and subharmonics. The QPO detected at ˜1.56 Hz drifts to a higher centroid frequency of ˜1.74 in the course of the observation, while the QPO detected at ˜6.6 Hz disappeared during hard flarings. The fractional rms at the QPO and the subharmonic frequencies increases with photon energy, while at the second harmonic frequencies the rms seems to be constant. In addition, we have observed soft time lag at QPO and subharmonic frequencies up to a timescale of ˜35 ms; however, at the second harmonic frequencies there is weak/zero time lag. We attempt spectral modeling of the broadband data in the 0.7-25 keV band using the doubly absorbed disk plus thermal Comptonization model. Based on the spectral and timing properties, we identified the source to be in the hard-intermediate state of black hole X-ray binaries. To quantitatively fit the energy- and frequency-dependent fractional rms and time lag, we use a single-zone fluctuation propagation model and discuss our results in the context of that model.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019ApJ...887..101J">2019ApJ...887..101J</a>
Title: Timing analysis of Swift J1658.2-4242's outburst in 2018 with Insight-HXMT, NICER and AstroSat
Authors: Xiao, G. C., Lu, Y., Ma, X., Ge, M. Y., Yan, L. L., Li, Z. J., Tuo, Y. L., Zhang, Y., Zhang, W., Liu, H. X., Zhou, D. K., Zhang, L., Bu, Q. C., Cao, X. L., Jiang, W. C., Chen, Y. P., Zhang, S., Lu, F. J., Chen, L., Qu, J. L., Song, L. M., Zhang, S. N., Zhuang, R. L., Shang, R. C., & Jin, Y. J.
Bibliographic Code: 2019JHEAp..24...30X
Keywords: Accretion, accretion disks, Black hole physics, X-rays: binaries: Swift J1658.2-4242, Astrophysics - High Energy Astrophysical Phenomena
Abstract: We present the observational results from a detailed timing analysis of the black hole candidate Swift J1658.2-4242 during its 2018 outburst with the observations of Hard X-ray Modulation Telescope (Insight-HXMT), Neutron Star Interior Composition Explorer (NICER) and AstroSat in 0.1-250 keV. The evolution of intensity, hardness and integrated fractional root mean square (rms) observed by Insight-HXMT and NICER are presented in this paper. Type-C quasi-periodic oscillations (QPOs) observed by NICER (0.8-3.5 Hz) and Insight-HXMT (1-1.6 Hz) are also reported in this work. The features of the QPOs are analyzed with an energy range of 0.5-50 keV. The relations between QPO frequency and other characteristics such as intensity, hardness and QPO rms are carefully studied. The timing and spectral properties indicate that Swift J1658.2-4242 is a black hole binary system. Besides, the rms spectra of the source calculated from the simultaneous observation of Insight-HXMT, NICER and AstroSat support the Lense-Thirring origin of the QPOs. The relation between QPO phase lag and the centroid frequency of Swift J1658.2-4242 reveals a near zero constant when < 4Hz and a soft phase lag at 6.68 Hz. This independence follows the same trend as the high inclination galactic black hole binaries such as MAXI J1659-152.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019JHEAp..24...30X">2019JHEAp..24...30X</a>
Title: UVIT Open Cluster Study. II. Detection of Extremely Low Mass White Dwarfs and Post-Mass Transfer Binaries in M67
Authors: Jadhav, V. V., Sindhu, N., & Subramaniam, A.
Bibliographic Code: 2019ApJ...886...13J
Keywords: Blue straggler stars, Hertzsprung Russell diagram, Open star clusters, Ultraviolet astronomy, White dwarf stars, 1736, 1799, 168, 1160, 725, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies
Abstract: A detailed study of the UV-bright stars in the old open star cluster M67 is presented based on the far-UV observations using the Ultra Violet Imaging Telescope (UVIT) on ASTROSAT. The UV and UV-optical color-magnitude diagrams (CMDs), along with overlaid isochrones, are presented for the member stars, which include blue straggler stars (BSSs), triple systems, white dwarfs (WDs), and spectroscopic binaries. The CMDs suggest the presence of excess UV flux in many members, which could be extrinsic or intrinsic to them. We construct multiwavelength spectral energy distributions (SEDs) using photometric data from the UVIT, Gaia DR2, Two Micron All Sky Survey, and Wide-Field Infrared Survey Explorer surveys, along with optical photometry. We fitted model SEDs to seven WDs and find that four of them have mass >0.5 M <SUB>⊙</SUB> and cooling age of less than 200 Myr, thus demanding BSS progenitors. SED fits to 23 stars detect extremely low mass (ELM) WD companions to WOCS2007, WOCS6006, and WOCS2002 and a low-mass WD to WOCS3001, which suggest these to be post-mass transfer (MT) systems. Twelve sources with possible WD companions need further confirmation. Nine sources have X-ray and excess UV flux, possibly arising out of stellar activity. This study demonstrates that UV observations are key to detecting and characterizing the ELM WDs in nondegenerate systems, which are ideal test beds to explore the formation pathways of these peculiar WDs. The increasing detection of post-MT systems among BSSs and main-sequence stars suggests a strong MT pathway and stellar interactions in M67.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019ApJ...886...13J">2019ApJ...886...13J</a>
Title: Journey through the birth, growth and maturity of X-ray astronomy
Authors: Agrawal, P. C.
Bibliographic Code: 2019RAA....19..138A
Keywords:
Abstract: In this biographical account, I recall my scientific journey that began in 1961 when I joined as a Physics Trainee in the BARC Training School and after one year course was admitted in TIFR as a Research Associate. During 1962-1967, I worked on determination of the He<SUP>3</SUP>/He<SUP>4</SUP> ratio in cosmic rays using a gas Cerenkov counter and also used a scintillation + solid Cerenkov detector to measure flux of protons and helium nuclei in primary cosmic rays. I also developed a large area scintillation-Cerenkov detector with a spark chamber between them for measuring charge composition of primary cosmic rays. I joined the X-ray astronomy programme in 1967 initiated by Prof. B. V. Sreekantan and played a leading role in developing a balloon borne hard X-ray instrument for studying time variability and energy spectra of cosmic X-ray sources. We carried out a series of balloon experiments and studied temporal intensity variation and energy spectra of X-ray objects like Sco X-1, Cyg X-1, etc. Results on rapid intensity changes and spectra appeared in a series of publications based on which I obtained a Ph.D. degree in 1972. Immediately afterwards, I joined Prof. Gordon Garmire at Caltech as a Research Fellow (a post-doctoral position) to work with him on the low energy component of HEAO A-2 experiment. The instrument was developed and successfully realised. Working on this satellite experiment was a thrilling experience which exposed me to the technical complexities of space instruments. The HEAO-1 satellite was launched in 1978. During 1978-1979, Guenter Riegler and I, working at JPL, reported detection of intense soft X-rays from several classes of extragalactic and Galactic objects like new BL Lacs, a new AM Her binary, coronal X-rays from active stars, etc. After returning to TIFR, I developed a rocket experiment with K. P. Singh to map the spatial and spectral distribution of the Soft Diffuse X-ray Background. At the same time, I also designed 15 cm deep Xenon filled Proportional Counters (XPCs) of ∼ 2500 cm<SUP>2</SUP> area for balloon experiments which were done during 1983-1990 and produced new results on several X-ray binaries. During this period I also developed a proportional counter (PC) based satellite instrument and submitted a proposal jointly with the astronomy group at the Indian Space Research Organization (ISRO) for an Indian X-ray Astronomy Experiment (IXAE). After success of the Polar Satellite Launch Vehicle (PSLV), the IXAE instrument was included on the top deck of the IRS-P3 satellite that was launched in March 1996. Despite the severe limitation of a polar orbit, the IXAE observed a good number of X-ray binaries and produced interesting results, with the most notable being quasi-regular bursts from a new transient, GRS 1915+105, which turned out to be an enigmatic black hole source. Following success of the IXAE, a proposal for an ambitious multiwavelength astronomy satellite, named ASTROSAT, with broad spectral coverage in the optical, NUV, FUV, soft X-ray and hard X-ray by a suite of instruments, was submitted to ISRO and finally approved by the Govt. of India in 2004. Over the next 10 years, the development and fabrication of the five ASTROSAT instruments were accomplished and a PSLV launch on Sept. 28, 2015 placed ASTROSAT in a 650 km circular orbit with 6 degree inclination. The ASTROSAT instruments have been performing well for ∼ 4 years and have observed more than 500 cosmic sources. Results from ASTROSAT have appeared in a large number of journal publications. Successful realization and performance of ASTROSAT marks culmination of my research career. I formally retired as a Senior Professor from TIFR on 2006 April 30 on attaining the age of superannuation. However by agreement with TIFR, ISRO designated me ‘ISRO Chair Professor’, which was later renamed as ISRO Satish Dhawan Professor, and asked me to continue to lead ASTROSAT as PI. At the end of my tenure as ISRO Professor on 2011 April 30, I relinquished the PI position. I am at present affiliated with the MU-DAE Centre of Excellence for Basic Sciences, Mumbai University Campus at Mumbai as an Emeritus Professor.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019RAA....19..138A">2019RAA....19..138A</a>
Title: A variable-frequency HFQPO in GRS 1915+105 as observed with AstroSat
Authors: Belloni, T. M., Bhattacharya, D., Caccese, P., Bhalerao, V., Vadawale, S., & Yadav, J. S.
Bibliographic Code: 2019MNRAS.489.1037B
Keywords: accretion, accretion discs, black hole physics, relativistic processes, X-rays: binaries, X-rays: individual: GRS 1915+105, Astrophysics - High Energy Astrophysical Phenomena
Abstract: From the analysis of more than 92 ks of data obtained with the Large Area X-Ray Proportional Counter (LAXPC) instrument onboard AstroSat we have detected a clear high-frequency quasi-periodic oscillation (HFQPO) whose frequency varies between 67.4 and 72.3 Hz. In the classification of variability classes of GRS 1915+105, at the start of the observation period the source was in class ω and at the end the variability was that of class μ: both classes are characterized by the absence of hard intervals and correspond to disc-dominated spectra. After normalization to take into account time variations of the spectral properties as measured by X-ray hardness, the QPO centroid frequency is observed to vary along the hardness-intensity diagram, increasing with hardness. We also measure phase lags that indicate that HFQPO variability at high energies lags that at lower energies and detect systematic variations with the position on the hardness-intensity diagram. This is the first time that (small) variations of the HFQPO frequency and lags are observed to correlate with other properties of the source. We discuss the results in the framework of existing models, although the small (7 per cent) variability observed is too small to draw firm conclusions.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019MNRAS.489.1037B">2019MNRAS.489.1037B</a>
Title: DustKING - the story continues: dust attenuation in NGC628
Authors: Decleir, M., De Looze, I., Boquien, M., & Baes, M.
Bibliographic Code: 2019BSRSL..88..307D
Keywords: galaxies: ISM, NGC 628, dust, extinction, Astrophysics - Astrophysics of Galaxies
Abstract: Dust attenuation is a crucial but highly uncertain parameter that hampers the determination of intrinsic galaxy properties, such as stellar masses, star formation rates and star formation histories. The shape of the dust attenuation law is not expected to be uniform between galaxies, nor within a galaxy. Our DustKING project was introduced at the first BINA workshop in 2016 and aims to study the variations of dust attenuation curves in nearby galaxies. At the second BINA workshop in 2018, I presented the results of our pilot study for the spiral galaxy NGC628. We find that the average attenuation law of this galaxy is characterised by a MW-like bump and a steep UV slope. Furthermore, we observe intriguing variations within the galaxy, with regions of high A_V exhibiting a shallower attenuation curve. Finally, we discuss how our work might benefit from data taken with the UVIT from the Indian AstroSat mission.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019BSRSL..88..307D">2019BSRSL..88..307D</a>
Title: Three years of AstroSat: Indian Astronomy mission
Authors: Veerappa, G.
Bibliographic Code: 2019BSRSL..88..190V
Keywords: Mission: AstroSat, X-rays, UV
Abstract: The AstroSat is India’s first multi-wavelength satellite dedicated for astronomy. It carries five scientific payloads covering a wide wavelength range (from visual/ultraviolet to high energy X-rays) and an auxiliary payload for monitoring in-situ charged particles. In this paper we describe the mission, the scientific payloads and a few important results from the AstroSat. The AstroSat is a proposal driven mission open for international users. The data is made public after a lock-in period of twelve months. We give a detailed description of the procedure to access archival data of the AstroSat. AstroSat has successfully completed three years in orbit and continues to yield valuable science data.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019BSRSL..88..190V">2019BSRSL..88..190V</a>
Title: Short-term variabilities in X-ray/Optical/UV emission from Seyfert 1 galaxy NGC 4593
Authors: Naik, S., Pal, M., & Kumari, N.
Bibliographic Code: 2019BSRSL..88..143N
Keywords: accretion, accretion disks, active galactic nuclei, NGC4593, X-rays
Abstract: We present results obtained from a detailed multi-band analysis of the Seyfert 1 galaxy NGC 4593. Publicly available Swift monitoring observations in optical, ultraviolet and X-ray bands were used in the present work. A total of 185 pointings over a duration of about a month were used to understand the variabilities in emission in optical, ultraviolet, soft and hard X-ray bands. It was found that the source emission in the hard X-ray band (1.5-10 keV range) was highly variable compared to that at longer wavelengths. The variability amplitude decreased gradually from hard X-ray to optical bands. A cross-correlation analysis between light curves in different bands inferred that the observed variation in ultraviolet and optical bands is strongly correlated with the hard X-ray emission. Observed time lag for changes in emission in longer wavelength bands with respect to the hard X-ray emission suggests X-ray reprocessing as the cause of variation. Though the observed lag spectrum follows a 4/3rd power of wavelength, as predicted by the standard disk model, the estimated lags are found to be larger than the lags predicted from the standard disk model. This suggests that the actual disk in NGC 4593 is possibly larger than the standard thin disk. Though similar findings have been reported in NGC 5548 and Fairall 9, simultaneous near-infrared, ultraviolet, soft X-ray and hard X-ray band observations of a sample of AGNs with the 1.2 m telescope of Physical Research Laboratory at Mount Abu, India and first space-based multiwavelength observatory of India - AstroSat will provide significant information on the actual and model predicted sizes of the accretion disk.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019BSRSL..88..143N">2019BSRSL..88..143N</a>
Title: On the requirement for a high resolution spectrograph to investigate the multiplicity of massive stars with the 3.6-m Devasthal Optical Telescope
Authors: De Becker, M.
Bibliographic Code: 2019BSRSL..88..128D
Keywords: Massive stars, Spectroscopy, Binaries, Visible
Abstract: The investigation of the multiplicity of massive stars constitutes a key aspect of their understanding. On the one hand, the evolution of short period systems can be tremendously influenced by the companion, for instance through mass exchange or common envelope evolution. On the other hand, the wind-wind interaction region in massive multiple systems is the seat of a high level physics, including strong thermal X-ray emission, particle acceleration and non-thermal emission processes. The description and understanding of these processes require the accurate determination of orbital parameters, notably through spectroscopic studies. In this context, it is timely to anticipate some relevant applications of a high resolution spectrograph (HRS) to be mounted on the Devasthal Optical Telescope, emphasizing its potential complementarity with other Indian facilities such as the GMRT and Astrosat.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019BSRSL..88..128D">2019BSRSL..88..128D</a>
Title: Prompt Emission Polarimetry of Gamma-Ray Bursts with the AstroSat CZT Imager
Authors: Chattopadhyay, T., Vadawale, S. V., Aarthy, E., Mithun, N. P. S., Chand, V., Ratheesh, A., Basak, R., Rao, A. R., Bhalerao, V., Mate, S., Arvind, B., Sharma, V., & Bhattacharya, D.
Bibliographic Code: 2019ApJ...884..123C
Keywords: gamma-ray burst: general, gamma-ray burst: individual: 151006A, 160106A, 160131A, 160325A, 160509A, 160607A, 160623A, 160703A, instrumentation: detectors, polarization, X-rays: general, Astrophysics - High Energy Astrophysical Phenomena, Astrophysics - Instrumentation and Methods for Astrophysics
Abstract: X-ray and gamma-ray polarization measurements of the prompt emission of gamma-ray bursts (GRBs) are believed to be extremely important for testing various models of GRBs. So far, the available measurements of hard X-ray polarization of GRB prompt emission have not significantly constrained the GRB models, particularly because of the difficulty of measuring polarization in these bands. The CZT Imager (CZTI) on board AstroSat is primarily an X-ray spectroscopic instrument that also works as a wide-angle GRB monitor due to the transparency of its support structure above 100 keV. It also has experimentally verified polarization measurement capability in the energy range 100-300 keV and thus provides a unique opportunity to attempt spectropolarimetric studies of GRBs. Here we present the polarization data for the brightest 11 GRBs detected by CZTI during its first year of operation. Among these, five GRBs show polarization signatures with ⪆3σ, and one GRB shows 2σ detection significance. We place upper limits for the remaining five GRBs. We provide details of the various tests performed to validate our polarization measurements. While it is difficult yet to discriminate between various emission models with the current sample alone, the large number of polarization measurements that CZTI expects to gather in its minimum lifetime of five years should help to significantly improve our understanding of the prompt emission.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019ApJ...884..123C">2019ApJ...884..123C</a>
Title: Spectro-timing analysis of MAXI J1535-571 using AstroSat
Authors: Bhargava, Y., Belloni, T., Bhattacharya, D., & Misra, R.
Bibliographic Code: 2019MNRAS.488..720B
Keywords: accretion, accretion discs, black hole physics, X-rays: binaries, Astrophysics - High Energy Astrophysical Phenomena
Abstract: We report the results of the analysis of an AstroSat observation of the black hole candidate MAXI J1535-571 during its hard-intermediate state. We studied the evolution of the spectral and timing parameters of the source during the observation. The observation covered a period of ∼5 d and consisted of 66 continuous segments, corresponding to individual spacecraft orbits. Each segment was analysed independently. The source count rate increased roughly linearly by ∼30 per cent. We modelled the spectra as a combination of radiation from a thermal disc component and a power law. The timing analysis revealed the presence of strong quasi-periodic oscillations with centroid frequency ν<SUB>QPO</SUB> fluctuating in the range of 1.7-3.0 Hz. We found a tight correlation between the QPO centroid frequency ν<SUB>QPO</SUB> and the power-law spectral index Γ, while ν<SUB>QPO</SUB> appeared not to be correlated with the linearly increasing flux itself. We discuss the implications of these results on physical models of accretion.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019MNRAS.488..720B">2019MNRAS.488..720B</a>
Title: Activity of the Red Dwarf FR Cnc from Observations with a Robotic IRT-20 Telescope at the Zvenigorod Observatory of INASAN
Authors: Savanov, I. S., Naroenkov, S. A., Nalivkin, M. A., Pandey, J. C., & Karmakar, S.
Bibliographic Code: 2019AstL...45..602S
Keywords: Variable stars
Abstract: We present the results of our new photometric observations of the chromospherically active star FR Cnc performed with a robotic telescope at the Zvenigorod Observatory of INASAN in March 2019. The observations were carried out from March 2 to 13, 2019, in the period of time that coincided most closely in time interval with the observations of this star with the Astrosat space telescope. A total of 450 magnitude estimates were obtained in each of the B, V, and R photometric bands over 12 days (from HJD 2458545.2 to HJD 2458556.5). Based on the V-band photometric variability of the star, we have constructed a map of surface temperature inhomogeneities. According to our estimate, the spot area on the surface of the star was 12% of its total surface area in early 2019. On March 3, 2019, a flare on FR Cnc was detected during our observations at a time corresponding to HJD = 2458554.25891447 or rotation phase 0.8. The data for the flare suggest a rapid brightening of the star by 0.19<SUP> m </SUP> in the B band and by 0.09<SUP> m </SUP> in the V band; the flare lasted for about 60 min. The energy radiated in the flare was ITE<SUB>B</SUB> = 2.17 × 10<SUP>33</SUP> erg and Ey = 1.40 × 10<SUP>33</SUP> erg in the B and V bands, respectively.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019AstL...45..602S">2019AstL...45..602S</a>
Title: Time-varying Polarized Gamma-Rays from GRB 160821A: Evidence for Ordered Magnetic Fields
Authors: Sharma, V., Iyyani, S., Bhattacharya, D., Chattopadhyay, T., Rao, A. R., Aarthy, E., Vadawale, S. V., Mithun, N. P. S., Bhalerao, V. B., Ryde, F., & Pe'er, A.
Bibliographic Code: 2019ApJ...882L..10S
Keywords: Gamma-ray bursts, High energy astrophysics, Magnetic fields, 629, 739, 994, Astrophysics - High Energy Astrophysical Phenomena
Abstract: GRB 160821A is the third most energetic gamma-ray burst observed by the Fermi gamma-ray space telescope. Based on the observations made by the Cadmium Zinc Telluride Imager on board AstroSat, here we report the most conclusive evidence to date of (i) high linear polarization ({66}<SUB>-27</SUB><SUP>+26</SUP> % ;5.3σ detection), and (ii) variation of polarization angle with time, occurring twice during the rise and decay phase of the burst at 3.5σ and 3.1σ detections, respectively. All confidence levels are reported for two parameters of interest. These observations strongly suggest synchrotron radiation produced in magnetic field lines that are highly ordered on angular scales of 1/Γ, where Γ is the Lorentz factor of the outflow.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019ApJ...882L..10S">2019ApJ...882L..10S</a>
Title: UVIT Open Cluster Study. I. Detection of a White Dwarf Companion to a Blue Straggler in M67: Evidence of Formation through Mass Transfer
Authors: Sindhu, N., Subramaniam, A., Jadhav, V. V., Chatterjee, S., Geller, A. M., Knigge, C., Leigh, N., Puzia, T. H., Shara, M., & Simunovic, M.
Bibliographic Code: 2019ApJ...882...43S
Keywords: open clusters and associations: individual: M67, stars: individual: blue stragglers, white dwarfs, ultraviolet: stars, Astrophysics - Solar and Stellar Astrophysics
Abstract: The old open cluster M67, populated with blue straggler stars (BSSs), is a well-known test bed to study the BSS formation pathways. Here, we report the first direct detection of a white dwarf (WD) companion to a BSS in M67, using far-UV images from the Ultra-Violet Imaging telescope on ASTROSAT. Near-simultaneous observations in three far-UV bands combined with Galaxy Evolution Explorer, International Ultraviolet Explorer, and ground- and space-based photometric data covering a 0.14-11.5 μm range for WOCS1007 were found to require a binary fit to its spectral energy distribution (SED), consisting of a BSS and a hot companion. On the other hand, a single spectral fit was found to be satisfactory for the SEDs of two other BSSs, WOCS1006 and WOCS2011, with the latter showing a deficient far-UV flux. The hot companion of WOCS1007 is found to have a {T}<SUB>eff</SUB>} ˜ 13,250-13,750 K and a radius of 0.09 ± 0.01 {R}<SUB>⊙ </SUB>. A comparison with WD models suggests it to be a low-mass WD (˜0.18{M}<SUB>⊙ </SUB>), in agreement with the kinematic mass from the literature. As a low-mass WD (<0.4{M}<SUB>⊙ </SUB>) necessitates formation through mass transfer in close binaries, WOCS1007 with a known period of 4.2 days along with its fast rotation, is likely to be formed by a case A or case B binary evolution.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019ApJ...882...43S">2019ApJ...882...43S</a>
Title: Thermal evolution and quiescent emission of transiently accreting neutron stars
Authors: Potekhin, A. Y., Chugunov, A. I., & Chabrier, G.
Bibliographic Code: 2019A&A...629A..88P
Keywords: stars: neutron, X-rays: binaries, Astrophysics - High Energy Astrophysical Phenomena
Abstract:
Aims: We study the long-term thermal evolution of neutron stars in soft X-ray transients (SXTs), taking the deep crustal heating into account consistently with the changes of the composition of the crust. We collect observational estimates of average accretion rates and thermal luminosities of such neutron stars and compare the theory with observations.
Methods: We performed simulations of thermal evolution of accreting neutron stars, considering the gradual replacement of the original nonaccreted crust by the reprocessed accreted matter, the neutrino and photon energy losses, and the deep crustal heating due to nuclear reactions in the accreted crust. We also tested and compared results for different modern theoretical models. We updated a compilation of the observational estimates of the thermal luminosities in quiescence and average accretion rates in the SXTs and compared the observational estimates with the theoretical results.
Results: The long-term thermal evolution of transiently accreting neutron stars is nonmonotonic. The quasi-equilibrium temperature in quiescence reaches a minimum and then increases toward the final steady state. The quasi-equilibrium thermal luminosity of a neutron star in an SXT can be substantially lower at the minimum than in the final state. This enlarges the range of possibilities for theoretical interpretation of observations of such neutron stars. The updates of the theory and observations leave the previous conclusions unchanged, namely that the direct Urca process operates in relatively cold neutron stars and that an accreted heat-blanketing envelope is likely present in relatively hot neutron stars in the SXTs in quiescence. The results of the comparison of theory with observations favor suppression of the triplet pairing type of nucleon superfluidity in the neutron-star matter.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019A&A...629A..88P">2019A&A...629A..88P</a>
Title: AstroSat observation of GX 5-1: spectral and timing evolution
Authors: Bhulla, Y., Misra, R., Yadav, J. S., & Jaaffrey, S. N. A.
Bibliographic Code: 2019RAA....19..114B
Keywords: Astrophysics - High Energy Astrophysical Phenomena
Abstract: We report on the first analysis of an AstroSat observation of the Z-source GX 5-1 on 2017 February 26-27. The hardness-intensity plot reveals that the source traced out the horizontal and normal branches. The 0.8-20 keV spectra from simultaneous SXT and LAXPC data at different locations of the hardness-intensity plot can be described well by disk emission and a thermal Comptonized component. The ratio of the disk flux to the total flux, i.e., the disk flux ratio, increases monotonically along the horizontal branch to the normal one. Thus, the difference between the normal and horizontal branches is that in the normal branch, the disk dominates the flux while in the horizontal one it is the Comptonized component which dominates. The disk flux scales with the inner disk temperature as {T}<SUB>in</SUB>}<SUP>5.5</SUP> and not as {T}<SUB>in</SUB>}<SUP>4</SUP>, suggesting that either the inner radius changes dramatically or that the disk is irradiated by the thermal component changing its hardness factor. The power spectra reveal a quasi-periodic oscillation (QPO) whose frequency changes from ∼30 Hz to 50 Hz. The frequency is found to correlate well with the disk flux ratio. In the 3-20 keV LAXPC band, the r.m.s. of the QPO increases with energy (r.m.s. \propto {E}<SUP>0.8</SUP>), while the harder X-rays seem to lag the soft ones with a time-delay of milliseconds. The results suggest that the spectral properties of the source are characterized by the disk flux ratio and that the QPO has its origin in the corona producing the thermal Comptonized component.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019RAA....19..114B">2019RAA....19..114B</a>
Title: Broad-band reflection spectroscopy of MAXI J1535-571 using AstroSat: estimation of black hole mass and spin
Authors: Sridhar, N., Bhattacharyya, S., Chandra, S., & Antia, H. M.
Bibliographic Code: 2019MNRAS.487.4221S
Keywords: accretion, accretion discs, black hole physics, methods: data analysis, X-rays: binaries, Astrophysics - High Energy Astrophysical Phenomena, High Energy Physics - Experiment, Physics - Data Analysis, Statistics and Probability
Abstract: We report the results from AstroSat observations of the transient Galactic black hole X-ray binary MAXI J1535-571 during its hard-intermediate state of the 2017 outburst. We systematically study the individual and joint spectra from two simultaneously observing AstroSat X-ray instruments, and probe and measure a number of parameter values of accretion disc, corona, and reflection from the disc in the system using models with generally increasing complexities. Using our broad-band (1.3-70 keV) X-ray spectrum, we clearly show that a soft X-ray instrument, which works below 10-12 keV, alone cannot correctly characterize the Comptonizing component from the corona, thus highlighting the importance of broad-band spectral analysis. By fitting the reflection spectrum with the latest version of the RELXILL family of relativistic reflection models, we constrain the black hole's dimensionless spin parameter to be 0.67^{+0.16}_{-0.04}. We also jointly use the reflection spectral component (RELXILL) and a general relativistic thin disc component (Kerrbb), and estimate the black hole's mass and distance to be 10.39_{-0.62}^{+0.61} M_{\odot } and 5.4_{-1.1}^{+1.8} kpc, respectively.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019MNRAS.487.4221S">2019MNRAS.487.4221S</a>
Title: AstroSat view of MAXI J1535-571: broad-band spectro-temporal features
Authors: Sreehari, H., Ravishankar, B. T., Iyer, N., Agrawal, V. K., Katoch, T. B., Mandal, S., & Nandi, A.
Bibliographic Code: 2019MNRAS.487..928S
Keywords: accretion, black hole physics, stars: individual: MAXI J1535-571, X-rays: binaries, Astrophysics - High Energy Astrophysical Phenomena
Abstract: We present the results of Target of Opportunity (ToO) observations made with AstroSat of the newly discovered black hole binary MAXI J1535-571. We detect prominent C-type quasi-periodic oscillations (QPOs) of frequencies varying from 1.85 to 2.88 Hz, along with distinct harmonics in all the AstroSat observations. We note that while the fundamental QPO is seen in the 3-50 keV energy band, the harmonic is not significant above ∼35 keV. The AstroSat observations were made in the hard intermediate state, as seen from state transitions observed by MAXI and Swift. We attempt spectral modelling of the broad-band data (0.7-80 keV) provided by AstroSat using phenomenological and physical models. The spectral modelling using nthComp gives a photon index in the range 2.18-2.37 and electron temperature ranging from 21 to 63 keV. The seed photon temperature is within 0.19 to 0.29 keV. The high flux in 0.3-80 keV band corresponds to a luminosity varying from 0.7 to 1.07 L<SUB>Edd</SUB> assuming the source to be at a distance of 8 kpc and hosting a black hole with a mass of 6 M<SUB>☉</SUB>. The physical model based on the two-component accretion flow gives disc accretion rates as high as ∼ 1 \dot{m}_{ Edd} and halo rate ∼ 0.2 \dot{m}_{ Edd}, respectively. The near-Eddington accretion rate seems to be the main reason for the unprecedented high flux observed from this source. The two-component spectral fitting of AstroSat data also provides an estimate of a black hole mass between 5.14 and 7.83 M<SUB>☉</SUB>.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019MNRAS.487..928S">2019MNRAS.487..928S</a>
Title: Pulse Phase Variation of the Cyclotron Line in HMXB 4U 1907+09 with AstroSat LAXPC
Authors: Varun, Pradhan, P., Maitra, C., Raichur, H., & Paul, B.
Bibliographic Code: 2019ApJ...880...61V
Keywords: binaries: spectroscopic, methods: data analysis, pulsars: individual: 4U 1907+09, radiation mechanisms: non-thermal, stars: magnetic field, X-rays: binaries
Abstract: We present timing and spectral analysis of data from an observation of the high mass X-ray binary pulsar 4U 1907+09 with the Large Area X-ray Proportional Counter instrument onboard AstroSat. The light curve consisted of a flare at the beginning of the observation, followed by persistent emission. The pulsar continues to spin down, and the pulse profile is found to be double-peaked up to 16 keV with the peaks separated by a phase of ∼0.45. Significant energy dependence of the pulse profile is seen with diminishing amplitude of the secondary peak above 16 keV, and increasing amplitude of the main peak up to 40 keV with a sharp decline after that. We confirm earlier detections of the cyclotron resonance scattering feature (CRSF) in 4U 1907+09 at ∼18.5 ± 0.2 keV in the phase-averaged spectrum with a high detection significance. An intensity resolved spectral analysis of the initial flare in the light curve shows that the CRSF parameters do not change with a change in luminosity by a factor of 2.6. We also performed pulse phase-resolved spectral analysis with 10 independent phase bins. The energy and the strength of the cyclotron line show pulse phase dependence that is in agreement with previous measurements. Two features from the current observation (different energy dependence of the two pulse peaks and a strong CRSF only around the secondary peak) both indicate a deviation from a dipole geometry of the magnetic field of the neutron star, or complex beaming pattern from the two poles.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019ApJ...880...61V">2019ApJ...880...61V</a>
Title: Soft x-ray tomographic reconstruction of Heliotron J plasma for the study of magnetohydrodynamic equilibrium and stability
Authors: Purohit, S., Suzuki, Y., Ohdachi, S., & Yamamoto, S.
Bibliographic Code: 2019PlST...21f5102P
Keywords: Heliotron J, tomography, soft x-ray, magnetohydrodynamics
Abstract: The preparation for an experimental soft x-ray tomography study on the Heliotron J (H-J) machine is carried out, with the objectives of evaluating the capability of the current soft x-ray tomographic system in terms of the identification of different mode structures and their poloidal rotation, and the axis shift with different plasma and machine parameters, and fixing the physics goals for the experimental study. These preparations were carried out via a simulated soft x-ray data set arising from different plasma conditions, such as magnetic islands, low beta and high beta. Soft x-ray tomography (SXT) is performed by the discrete pixel method including singular value decomposition and Phillips-Tikhonov regularization, to obtain clear and smooth images. The H-J soft x-ray tomography results from simulated soft x-rays for the equilibrium H-J plasma sensed the magnetic axis shift clearly and an estimate was also achieved. Successful reconstruction for mode structure m = 1/n = 1 was obtained along with the realization of the poloidal rotation of the structure. The reconstruction for the m = 2/n = 1 mode was not very clear for the current soft x-ray diagnostic design. Effective mode identification was not possible due to the lack of measurements. The SXT from the current soft x-ray diagnostic on H-J, the magnetic axis shift can be estimated and the m = 1/n = 1 mode can be studied. Study of higher poloidal modes is difficult with the current design.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019PlST...21f5102P">2019PlST...21f5102P</a>
Title: Microlensing of x-ray pulsars: A method to detect primordial black hole dark matter
Authors: Bai, Y., & Orlofsky, N.
Bibliographic Code: 2019PhRvD..99l3019B
Keywords: Astrophysics - High Energy Astrophysical Phenomena, Astrophysics - Astrophysics of Galaxies, Astrophysics - Instrumentation and Methods for Astrophysics, High Energy Physics - Phenomenology
Abstract: Primordial black holes (PBHs) with a mass from 10<SUP>-16</SUP> to 10<SUP>-11</SUP> M<SUB>☉</SUB> may comprise 100% of dark matter. Due to a combination of wave and finite source size effects, the traditional microlensing of stars does not probe this mass range. In this paper, we point out that x-ray pulsars with higher photon energies and smaller source sizes are good candidate sources for microlensing for this mass window. Among the existing x-ray pulsars, the Small Magellanic Cloud (SMC) X-1 source is found to be the best candidate because of its apparent brightness and long distance from Earth. We have analyzed the existing observation data of SMC X-1 by the RXTE telescope (around 10 days) and found that PBH as 100% of dark matter is close to but not yet excluded. Future longer observation of this source by x-ray telescopes with larger effective areas such as AstroSat, Athena, Lynx, and eXTP can potentially close the last mass window where PBHs can make up all of dark matter.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019PhRvD..99l3019B">2019PhRvD..99l3019B</a>
Title: A stochastic propagation model to the energy dependent rapid temporal behaviour of Cygnus X-1 as observed by AstroSat in the hard state
Authors: Maqbool, B., Mudambi, S. P., Misra, R., Yadav, J. S., Gudennavar, S. B., Bubbly, S. G., Rao, A., Jogadand, S., Patil, M. K., Bhattacharyya, S., & Singh, K. P.
Bibliographic Code: 2019MNRAS.486.2964M
Keywords: accretion, accretion discs, black hole physics, X-rays: binaries, X-rays: individual: Cygnus X-1, Astrophysics - High Energy Astrophysical Phenomena
Abstract: We report the results from analysis of six observations of Cygnus X-1 by Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) onboard AstroSat, when the source was in the hard spectral state as revealed by the broad-band spectra. The spectra obtained from all the observations can be described by a single-temperature Comptonizing region with disc and reflection components. The event mode data from LAXPC provides unprecedented energy dependent fractional root mean square (rms) and time-lag at different frequencies which we fit with empirical functions. We invoke a fluctuation propagation model for a simple geometry of a truncated disc with a hot inner region. Unlike other propagation models, the hard X-ray emission (>4 keV) is assumed to be from the hot inner disc by a single-temperature thermal Comptonization process. The fluctuations first cause a variation in the temperature of the truncated disc and then the temperature of the inner disc after a frequency dependent time delay. We find that the model can explain the energy dependent rms and time-lag at different frequencies.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019MNRAS.486.2964M">2019MNRAS.486.2964M</a>
Title: ICE SXT vs. ICESh95: Co-existence of Integrative and Conjugative Elements and Competition for a New Host
Authors: Parmeciano DI Noto, G., Iriarte, A., Ramírez, M. S., Centrón, D., & Quiroga, C.
Bibliographic Code: 2019NatSR...9.8045P
Keywords:
Abstract: Integrative and conjugative elements (ICEs) are mobile genetic elements that contribute to horizontal gene transfer. The aim of this work was to study different types of ICEs in clinical isolates of the emergent pathogen Shewanella spp., to compare their transfer efficiency and their ability to integrate a new host. Here we show that 3 out of 10 clinical isolates contained an ICE. Two of these elements were similar to ICEs from the SXT/R391 family and the other one was similar to ICESh95, a hybrid platform. Mating assays showed that these elements co-exist for several generations in the same host. Furthermore, transfer rates and competition assays between ICESh95 and ICESh392, an SXT-like element, suggest that the latter has evolved into a well-oiled machine that efficiently spread to different bacteria. Our results provide strong evidence of the role that ICEs play in the dissemination of genetic traits in nature and the implications that they have in the global threat of antimicrobial resistance.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019NatSR...9.8045P">2019NatSR...9.8045P</a>
Title: Systemic inflammation is associated with malaria and preterm birth in women living with HIV on antiretrovirals and co-trimoxazole
Authors: McDonald, C. R., Weckman, A. M., Conroy, A. L., Olwoch, P., Natureeba, P., Kamya, M. R., Havlir, D. V., Dorsey, G., & Kain, K. C.
Bibliographic Code: 2019NatSR...9.6758M
Keywords:
Abstract: Women living with HIV (WLHIV) have an increased risk of malaria in pregnancy (MiP). It is unclear if MiP in WLHIV causes a systemic inflammatory response and increases the risk of adverse birth outcomes, especially for women receiving antiretroviral therapy (ART) and daily trimethoprim-sulfamethoxazole (TMP/SXT). We analyzed repeated plasma samples in a cohort of malaria-exposed Ugandan WLHIV receiving ART and daily TMP/SXT to examine changes in inflammatory markers across pregnancy and their association with birth outcomes. Concentrations of CHI3L1, CRP, IL-18BP, IL-6, sICAM-1, and sTNFR2 were quantified by ELISA in 1115 plasma samples collected over pregnancy from 326 women. MiP was associated with increased sTNFR2, sICAM-1 and IL-18BP concentrations across pregnancy. Women who delivered preterm had elevated concentrations of sTNFR2 and altered levels of IL-6 during pregnancy. Women with sTNFR2 concentrations in the highest quartile within 6 weeks of delivery had an increased relative risk of preterm birth. Our results indicate that despite daily TMP/SXT, MiP in WLHIV induced a systemic inflammatory response that was associated with an increased risk of preterm birth. These findings highlight the need for additional strategies to protect WLHIV from malaria infection in pregnancy to promote healthy outcomes for mother and child.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019NatSR...9.6758M">2019NatSR...9.6758M</a>
Title: Investigation of the globular cluster NGC 2808 with the Ultra-Violet Imaging Telescope
Authors: Jain, R., Vig, S., & Ghosh, S. K.
Bibliographic Code: 2019MNRAS.485.2877J
Keywords: techniques: photometric, Hertzsprung-Russell and colour-magnitude diagrams, stars: horizontal branch, globular clusters: individual: NGC 2808, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies
Abstract: Globular clusters represent stellar laboratories where observations can be used to validate models of stellar evolution. In this study, we put forth new ultraviolet (UV) photometric results of stars in the Galactic globular cluster NGC 2808. NGC 2808 is known to host multiple stellar populations that include at least four distinct groups of horizontal branch (HB) stars. We have observed this cluster with the AstroSat-UltraViolet Imaging Telescope in two far-UV (FUV) and five near-UV (NUV) filters, respectively. These UV filters enable the identification of HB populations of stars. The results from four NUV filters exhibit bimodal distributions in magnitude histograms. The nature of bimodality has been investigated on the basis of distinct stellar types contributing to those bands. The colour-magnitude diagrams constructed using FUV and NUV filters enable the location of hot stellar populations, viz. stars belonging to red HB (RHB), blue HB, extreme HB, blue hook branch, and post-asymptotic giant branch. Prominent gaps are observed in the UV colour-magnitude diagrams. We report for the first time a photometric gap in an NUV colour-magnitude diagram that segregates the RHB population of this cluster into two groups that are likely to be associated with distinct generations of stars. We also investigate the spatial density distributions of various groups of stars in the cluster and comment on the proposed formation models of multiple populations.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019MNRAS.485.2877J">2019MNRAS.485.2877J</a>
Title: Detection of a White Dwarf Companion to a Blue Straggler Star in the Outskirts of Globular Cluster NGC 5466 with the Ultraviolet Imaging Telescope (UVIT)
Authors: Sahu, S., Subramaniam, A., Simunovic, M., Postma, J., Côté, P., Kameswera Rao, N., Geller, A. M., Leigh, N., Shara, M., Puzia, T. H., & Stetson, P. B.
Bibliographic Code: 2019ApJ...876...34S
Keywords: blue stragglers, globular clusters: individual: NGC 5466, binaries: general, ultraviolet: stars, white dwarfs, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies
Abstract: We report the discovery of a hot white dwarf (WD) companion to a blue straggler star (BSS) in the globular cluster (GC) NGC 5466, based on observations from the Ultra-Violet Imaging Telescope (UVIT) on board AstroSat. The Spectral Energy Distribution (SED) of the Far-UV detected BSS NH 84 was constructed by combining the flux measurements from four filters of UVIT, with GALEX, GAIA, and other ground-based observations. The SED of NH 84 reveals the presence of a hot companion to the BSS. The temperature and radius of the BSS (T {}<SUB>eff</SUB>}={8000}<SUB>-250</SUB><SUP>+1000</SUP> K, R/R {}<SUB>⊙ </SUB>=1.44+/- 0.05) derived from Gemini spectra and SED fitting using Kurucz atmospheric models are consistent with each other. The temperature and radius of the hotter companion of NH 84 (T {}<SUB>eff</SUB>}={{32,000}}+/- 2000 K, R/R {}<SUB>⊙ </SUB>=0.021+/- 0.007) derived by fitting Koester WD models to the SED suggest that it is likely to be a hot WD. The radial velocity derived from the spectra along with the proper motion from GAIA DR2 confirms NH 84 to be a kinematic member of the cluster. This is the second detection of a BSS-WD candidate in a GC, and the first in the outskirts of a low-density GC. The location of this BSS in NGC 5466 along with its dynamical age supports the mass-transfer pathway for BSS formation in low-density environments.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019ApJ...876...34S">2019ApJ...876...34S</a>
Title: Antimicrobial agent susceptibilities of Legionella pneumophila MLVA-8 genotypes
Authors: Sharaby, Y., Nitzan, O., Brettar, I., Höfle, M. G., Peretz, A., & Halpern, M.
Bibliographic Code: 2019NatSR...9.6138S
Keywords:
Abstract: Legionella pneumophila causes human lung infections resulting in severe pneumonia. High-resolution genotyping of L. pneumophila isolates can be achieved by multiple-locus variable-number tandem-repeat analysis (MLVA-8). Legionella infections in humans occur as a result of inhalation of bacteria-containing aerosols, thus, our aim was to study the antimicrobial susceptibilities of different MLVA-8 genotypes to ten commonly used antimicrobial agents in legionellosis therapy. Epidemiological cut-off values were determined for all antibiotics. Significant differences were found between the antimicrobial agents' susceptibilities of the three studied environmental genotypes (Gt4, Gt6, and Gt15). Each genotype exhibited a significantly different susceptibility profile, with Gt4 strains (Sequence Type 1) significantly more resistant towards most studied antimicrobial agents. In contrast, Gt6 strains (also Sequence Type 1) were more susceptible to six of the ten studied antimicrobial agents compared to the other genotypes. Our findings show that environmental strains isolated from adjacent points of the same water system, exhibit distinct antimicrobial resistance profiles. These differences highlight the importance of susceptibility testing of Legionella strains. In Israel, the most extensively used macrolide for pneumonia is azithromycin. Our results point at the fact that clarithromycin (another macrolide) and trimethoprim with sulfamethoxazole (SXT) were the most effective antimicrobial agents towards L. pneumophila strains. Moreover, legionellosis can be caused by multiple L. pneumophila genotypes, thus, the treatment approach should be the use of combined antibiotic therapy. Further studies are needed to evaluate specific antimicrobial combinations for legionellosis therapy.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019NatSR...9.6138S">2019NatSR...9.6138S</a>
Title: Probing the Cyclotron line characteristics of 4U 1538-522 using AstroSat-LAXPC
Authors: Varun, Maitra, C., Pragati, P., Harsha, R., & Biswajit, P.
Bibliographic Code: 2019MNRAS.484L...1V
Keywords: binaries : general, radiation mechanism : non-thermal, (stars:) pulsars : general, (stars:) pulsars : individual : 4U 1538-522, X-rays : stars, Astrophysics - High Energy Astrophysical Phenomena
Abstract: We report the first report on cyclotron line studies with the LAXPC instrument onboard AstroSat of the high-mass X-ray binary pulsar 4U 1538-52. During the observation of source that spanned about 1 d with a net exposure of 50 ks, the source X-ray flux remained constant. Pulse profile is double peaked in low-energy range and has a single peak in high-energy range, the transition taking place around the cyclotron line energy of the source. Cyclotron scattering feature (CRSF) is detected at ∼22 keV with a very high significance in phase-averaged spectrum. It is one of the highest signal to noise ratio detections of CRSF for this source. We performed detailed pulse-phase-resolved spectral analysis with 10 independent phase bins. We report the results of pulse-phase-resolved spectroscopy of the continuum and CRSF parameters. The cyclotron line parameters show pulse phase dependence over the entire phase with a CRSF energy variation of {∼ }13{{ per cent}}, which is in agreement with previous studies. We also confirm the increase in the centroid energy of the CRSF observed between the 1996-2004 (RXTE) and the 2012 (Suzaku) observations, reinforcing that the increase was a long-term change.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019MNRAS.484L...1V">2019MNRAS.484L...1V</a>
Title: AstroSat-CZTI Detection of Variable Prompt Emission Polarization in GRB 171010A
Authors: Chand, V., Chattopadhyay, T., Oganesyan, G., Rao, A. R., Vadawale, S. V., Bhattacharya, D., Bhalerao, V. B., & Misra, K.
Bibliographic Code: 2019ApJ...874...70C
Keywords: gamma-ray burst: general, gamma-ray burst: individual: GRB 171010A, polarization, radiation mechanisms: non-thermal, Astrophysics - High Energy Astrophysical Phenomena
Abstract: We present a spectropolarimetric analysis of GRB 171010A using data from AstroSat, Fermi, and Swift to provide insights into the physical mechanisms of the prompt radiation and jet geometry. The prompt emission from GRB 171010A was very bright (fluence >10<SUP>-4</SUP> erg cm<SUP>-2</SUP>) and had a complex structure that was composed of the superimposition of several pulses. The energy spectra deviate from the typical Band function to show a low-energy peak ∼15 keV, which we interpret as a power law with two breaks, with a synchrotron origin. Alternatively, the prompt spectra can also be interpreted as Comptonized emission, or a blackbody combined with a Band function. A time-resolved analysis confirms the presence of the low-energy component, while the peak energy is found to be confined in the range of 100-200 keV. The afterglow emission detected by Fermi-LAT is typical of an external shock model, and we constrain the initial Lorentz factor using the peak time of the emission. Swift-XRT measurements of the afterglow show an indication for a jet break, allowing us to constrain the jet opening angle to <6°. The detection of a large number of Compton-scattered events by AstroSat-CZTI provides an opportunity to study the hard X-ray polarization of the prompt emission. We find that the burst has a high time-variable polarization, and the emission has a higher polarization at energies above the peak energy. We discuss all observations in the context of GRB models and polarization arising as a result of physical or geometric effects: synchrotron emission from multiple shocks with ordered or random magnetic fields, a jet dominated by Poynting flux that undergoes abrupt magnetic dissipation, subphotospheric dissipation, a jet consisting of fragmented fireballs, and the Comptonization model.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019ApJ...874...70C">2019ApJ...874...70C</a>
Title: Thermonuclear X-ray bursts in rapid succession in 4U 1636-536 with AstroSat-LAXPC
Authors: Beri, A., Paul, B., Yadav, J. S., Antia, H. M., Agrawal, P. C., Manchanda, R. K., Dedhia, D., Chauhan, J. V., Pahari, M., Misra, R., Katoch, T., Madhwani, P., Shah, P., & Varun, M.
Bibliographic Code: 2019MNRAS.482.4397B
Keywords: accretion, accretion discs, stars: neutron, X-rays: binaries, X-rays: bursts, X-rays: individual: 4U 1636—536, Astrophysics - High Energy Astrophysical Phenomena
Abstract: We present results from an observation of the low-mass X-ray binary 4U 1636-536 obtained with the LAXPC instrument aboard AstroSat. The observations of 4U 1636-536 made during the performance verification phase of AstroSat showed seven thermonuclear X-ray bursts in a total exposure of ∼65 ks over a period of about two consecutive days. Moreover, the light curve of 4U 1636-536 revealed the presence of a rare triplet of X-ray bursts, having a wait time of about 5.5 min between the second and the third bursts. We also present results from time-resolved spectroscopy performed during these seven X-ray bursts. In addition, we have also detected a transient quasi-periodic oscillation at ∼5 Hz. However, we did not find any evidence of kilo-hertz quasi-periodic oscillations and/or X-ray burst oscillations, perhaps due to the hard spectral state of the source during this observation.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019MNRAS.482.4397B">2019MNRAS.482.4397B</a>
Title: LAXPC/AstroSat Study of ∼1 and ∼2 mHz Quasi-periodic Oscillations in the Be/X-Ray Binary 4U 0115+63 during Its 2015 Outburst
Authors: Roy, J., Agrawal, P. C., Iyer, N. K., Bhattacharya, D., Yadav, J. S., Antia, H. M., Chauhan, J. V., Choudhury, M., Dedhia, D. K., Katoch, T., Madhavani, P., Manchanda, R. K., Misra, R., Pahari, M., Paul, B., & Shah, P.
Bibliographic Code: 2019ApJ...872...33R
Keywords: pulsars: individual: 4U 0115+63, stars: emission-line, Be, X-rays: binaries, Astrophysics - High Energy Astrophysical Phenomena
Abstract: The Be/X-ray binary 4U 0115+63 was observed by the Large Area X-ray Proportional Counter (LAXPC) instrument on AstroSat on 2015 October 24 during the peak of a giant Type II outburst. Prominent intensity oscillations at ∼1 and ∼2 mHz frequency were detected during the outburst. Nuclear Spectroscopic Telescope Array (NuSTAR) observations made during the same outburst also show millihertz quasi-periodic oscillations (QPOs). Details of the oscillations and their characteristics deduced from LAXPC/AstroSat and NuSTAR observations are reported in this paper. Analysis of the archival Rossi X-ray Timing Explorer/Proportional Counter Array data during 2001-2011 also show the presence of millihertz QPOs during some of the outbursts, and details of these QPOs are also reported. Possible models to explain the origin of the millihertz oscillations are examined. Similar QPOs, albeit at higher frequencies, have been reported from other neutron star and black hole sources, and both may have a common origin. Current models to explain the instability in the inner accretion disk causing the intense oscillations are discussed.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019ApJ...872...33R">2019ApJ...872...33R</a>
Title: The Period Change of Cyg X-3
Authors: Antokhin, I. I., & Cherepashchuk, A. M.
Bibliographic Code: 2019ApJ...871..244A
Keywords: accretion, accretion disks, binaries: close, stars: individual: Cyg X-3, X-rays: binaries, Astrophysics - High Energy Astrophysical Phenomena, Astrophysics - Solar and Stellar Astrophysics
Abstract: By using available archival X-ray data, we significantly extended the list of times of X-ray minima. The new list includes 65 data points obtained by critically reanalyzing RXTE ASM data, 88 data points based on observations by MAXI, and 2 data points based on observations by SUZAKU and AstroSat. Analyzing the data along with times of X-ray minima available from the literature, we provide the most accurate estimate of the rate of period change to date. We do not confirm the existence of a second derivative of the orbital period suggested by some authors earlier. Instead, we find that the changes in the period can be fit by a sum of quadratic and sinusoidal functions. The period of sinusoidal variations is 15.79 yr. They can be related either to apsidal motion in the close binary with eccentricity e ≃ 0.03 or to the presence of a third body with a mass of about 0.7 M <SUB>☉</SUB> located at a distance ∼16 au from the close binary.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019ApJ...871..244A">2019ApJ...871..244A</a>
Title: AstroSat SXT Observations of Her X-1
Authors: Leahy, D. A., & Chen, Y.
Bibliographic Code: 2019ApJ...871..152L
Keywords: binaries: eclipsing, stars: individual: HZ Her&Her X-1, stars: neutron
Abstract: Two observations of the X-ray binary system Her X-1 by the AstroSat Soft X-ray Telescope were carried out in 2017. The first was during Low State and Turn-On, the second was during Main High State without and with dips. During Main High without dips, the power-law continuum, 1 keV emission line complex, and soft blackbody-like emission from Her X-1 are detected. Evidence is found in the spectrum for a highly ionized absorber, consistent with the recently detected corona in Her X-1. The X-ray spectra from different states are compared. The 6.4 keV fluorescent iron line is clearly detected in Low State. The different spectrum components are highly variable between states.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019ApJ...871..152L">2019ApJ...871..152L</a>
Title: Multi-Wavelength Observations of a Large Solar Flare
Authors: Chornogor, S. N., & Kondrashova, N. N.
Bibliographic Code: 2019OAP....32..114C
Keywords: active regions, solar flares, magnetic reconnections, multi-wavelength observations.
Abstract: We present the results of the multiwavelength study of the two-ribbon solar flare on July 19, 2000 in the active region NOAA 9087. The evolution and morphological properties of the flare productive active region have been analyzed. The active region was growing rapidly and showed a complex multipolar magnetic field configuration. It was large, producing many events, including the flare under consideration. The 3N/M6.4 two-ribbon flare was a prominent, long duration event in the active region evolution. According to Solar Geophysical Data (SGD) the flare lasted 2.5 hours. The flare energy release took place in many sites of the active region. We used combination of data from space and ground based observatories for study. The hard X-ray (HXR) and soft X-ray (SXR) data were obtained at the Yohkoh Telescopes (HXT and SXT) and Geostationary Operational Environmental Satellite (GOES). The fulldisk magnetograms and EUV-images were provided by the Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI) and Extreme ultraviolet Imaging Telescope (EIT). We used the H α filtergrams from the Meudon spectroheliograph and white light images of Big Bear Solar Observatory (BBSO). All the data show continuously evolving SXR, EUV and H α features during the flare. The HXR and the type III radio bursts were observed at the flare onset. The first H α flare kernels and the surge, connected with the filament eruption, were initiated near a large positive-polarity sunspot. The main bright kernels of the flare occurred at the centre of the active region near magnetic neutral line, after that the flare ribbons appeared along it. It was found that HXR coronal source was located along a magnetic polarity inversion line of the active region. EUV loop structures indicate the observational evidence of a magnetic reconnection during the main phase of the flare.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019OAP....32..114C">2019OAP....32..114C</a>
Title: UVIT-HST-GAIA view of NGC 288: a census of the hot stellar population and its properties from UV
Authors: Sahu, S., Subramaniam, A., Côté, P., Rao, N. K., & Stetson, P. B.
Bibliographic Code: 2019MNRAS.482.1080S
Keywords: stars: blue stragglers, stars: horizontal branch, stars: Hertzsprung-Russell and colour-magnitude diagrams, globular clusters: individual: NGC 288, ultraviolet: stars, stars: imaging, catalogues, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies
Abstract: A complete census of the blue horizontal branch (BHB) and blue straggler star (BSS) population within a 10' radius from the centre of the globular cluster NGC 288 is presented, based on images from the Ultraviolet Imaging Telescope (UVIT). The UV and UV-optical colour-magnitude diagrams (CMDs) are constructed by combining the UVIT, HST-ACS and ground data and are compared with the Bag of Stellar Tracks and Isochrones (BaSTI) isochrones generated for UVIT filters. We used stellar proper motion data from GAIA DR2 to select the cluster members. Our estimations of the temperature distribution of 110 BHB stars reveal two peaks, with the main peak at T<SUB>eff</SUB> ˜ 10 300 K, and with the distribution extending up to T<SUB>eff</SUB> ˜ 18 000 K. We identify the well-known photometric gaps, including the Grundahl jump (G-jump) in the BHB distribution, which are located between the peaks. We detect a plateau in the far-ultraviolet (FUV) magnitude for stars hotter than T<SUB>eff</SUB> ˜ 11 500 K (G-jump), which could be caused by atomic diffusion. We detect two extreme horizontal branch (EHB) candidates, with temperatures ranging from 29 000 to 32 000 K. The radial distribution of 68 BSSs suggests that the bright BSSs are more centrally concentrated than the faint BSSs and the BHB stars. We find that the BSSs have a mass range of 0.86-1.25 M<SUB>⊙</SUB> and an age range of 2-10 Gyr, with peaks at 1 M<SUB>⊙</SUB> and 4 Gyr respectively. This study showcases the importance of combining UVIT with HST, ground and GAIA data in deriving HB and BSS properties.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019MNRAS.482.1080S">2019MNRAS.482.1080S</a>
Title: Study of Timing Evolution from Nonvariable to Structured Large-amplitude Variability Transition in GRS 1915 + 105 Using AstroSat
Authors: Rawat, D., Pahari, M., Yadav, J. S., Jain, P., Misra, R., Bagri, K., Katoch, T., Agrawal, P. C., & Manchanda, R. K.
Bibliographic Code: 2019ApJ...870....4R
Keywords: accretion, accretion disks, black hole physics, X-rays: binaries, X-rays: individual: GRS 1915 + 105 LAXPC&AstroSat, Astrophysics - High Energy Astrophysical Phenomena
Abstract: In this work, we present a ∼90 ks continuous monitoring of the Galactic microquasar GRS 1915 + 105 with AstroSat when the source undergoes a major transition from a nonvariable, χ class (similar to radio-quiet χ class) to a structured, large-amplitude, periodic heartbeat state (similar to ρ class). We show that such a transition takes place via an intermediate state when the large-amplitude, irregular variability of the order of hundreds of seconds in the soft X-ray band turned into 100-150 s regular, structured, nearly periodic flares. The properties of strong low-frequency (LF) quasi-periodic oscillation (QPO) in the frequency range 3-5 Hz also evolve marginally during these variability transitions. We also study time-lag and rms spectra at the QPO and harmonic component and the dynamic power spectra. We note a few important differences between the heartbeat state and the ρ class. Interestingly, the time-averaged LF QPO properties in the hard X-ray band are relatively stable in three states when compared to the significant evolution observed in the slow variability properties at millihertz frequencies. Such relative stability of LF QPOs implies that the inner disk-corona coupled accretion flow, which determines the LF QPO properties, may be uninterrupted by the launch of long, large-amplitude flares.
URL: <a href="https://ui.adsabs.harvard.edu/abs/2019ApJ...870....4R">2019ApJ...870....4R</a>