Large Area X-ray Proportional Counter

Department of Astronomy & Astrophysics, TIFR

Launched on September 28, 2015

Here is a list of published papers from LAXPC Data:

Devasia et al. (2021). Thermonuclear X-ray bursts detected in Cyg X-2 using AstroSat/LAXPC. 2021NewA...8301479D 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 ×10-8 ergs/cm2, 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.

Singh et al. (2021). AstroSat soft X-ray observations of the symbiotic recurrent nova V3890 Sgr during its 2019 outburst. 2021MNRAS.501...36S 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.

Kumar et al. (2021). UV spectroscopy confirms SU Lyn to be a symbiotic star. 2021MNRAS.500L..12K 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.

Rubinur et al. (2021). A multiwavelength study of the dual nuclei in Mrk 212. 2021MNRAS.500.3908R 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.

Bhattacharya et al. (2021). Multiwavelength study of different flaring and low-activity states of blazar 4C+21.35. 2021MNRAS.500.1127B 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.

Tilak Katoch et al. (2020). AstroSatview of IGR J17091–3624 and GRS 1915+105:decoding the ‘pulse’ in the ‘Heartbeat State’. arXiv:2011.13282 arXiv:2011.13282v1 [astro-ph.HE] 26 Nov 2020MNRAS000,1–17(2020)Preprint 30 November 2020Compiled using MNRAS LATEX style file v3.0AstroSatview of IGR J17091–3624 and GRS 1915+105:decoding the ‘pulse’ in the ‘Heartbeat State’Tilak Katoch1⋆, Blessy E. Baby2,3, Anuj Nandi2, V. K. Agrawal2, H. M. Antia1andKallol Mukerjee1.1Department of Astronomy & Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai, 400005, India2Space Astronomy Group, ISITE Campus, U. R. Rao Satellite Centre, Outer Ring Road, Marathahalli, Bangalore, 560037, India3Department of Physics, University of Calicut, Malappuram,Kerala, 673635, India.Accepted XXX. Received YYY; in original form ZZZABSTRACTIGR J17091–3624 is a transient galactic black hole which has a distinctquasi-periodicvariability known as ‘heartbeat’, similar to the one observed in GRS 1915+105. Inthis paper, we report the results of∼125 ksAstroSatobservations of this sourceduring the 2016 outburst. For the first time a double peaked QPO (DPQ) is detectedin a few time segments of this source with a difference ofδf∼12 mHz between thetwo peaks. The nature of the DPQ was studied based on hardness ratios and usingthe 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 oftime along with harmonics (50−95 mHz). Broadband spectra in the range 0.7−23keV, obtained withSXTandLAXPC, could be fitted well with combination of athermal Comptonisation and a multicolour disc component model. DuringAstroSatobservation, the source was in the Soft-Intermediate State (SIMS) as observed withSwift/XRT. We present a comparative study of the ‘heartbeat’ state variability inIGR J17091–3624 with GRS 1915+105. Significant difference in the timing propertiesis observed although spectral parameters (Γ∼2.1−2.4 andTmax∼0.6−0.8 keV)in the broad energy band remain similar. Spectral properties of segments exhibitingSPQ and DPQ are further studied using simple phase resolved spectroscopy whichdoes not show a significant difference. Based on the model parameters, we obtain themaximum ratio of mass accretion rate in GRS 1915+105 to that in IGR J17091–3624as∼25 : 1. We discuss the implications of our findings and comment on the physicalorigin of these exotic variabilities.

Aru Beri et al. (2020). AstroSat Observations of the first Galactic ULX Pulsar Swift J0243.6+6124. arXiv:2010.08334 SwiftJ0243.6+6124, the first Galactic ultra-luminous X-ray pulsar, was observed during its 2017-2018 outburst with \emph{AstroSat} at both sub- and super-Eddington levels of accretionwith X-ray luminosities of LX∼7×1037 and 6×1038ergs−1, respectively.Our broadband timing and spectral observations show that X-ray pulsations at ∼9.85s have been detected up to 150keV 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% at 1.65keV to 40--80% at 70keV.The continuum X-ray spectra are well-modeled with a high energy cut-off power law(Γ∼0.6-0.7) and one or two blackbody components with temperatures of ∼0.35keV and 1.2keV, depending on the accretion this http URL iron line emission is observed at sub-Eddington level, while a broad emission feature at around 6.9keV is observed at the super-Eddington level, along with a blackbody radius(121−142km) that indicates the presence of optically thick outflows.

Sreehari et al. (2020). AstroSat view of GRS 1915+105 during the Soft State: Detection of HFQPOs and estimation of Mass and Spin arXiv:2010.03782 We report the results of AstroSat observations of GRS 1915+105 obtained using 100 ks guaranteed-time (GT) during the soft state. The Color-Color Diagram (CCD) indicates a variability class of δ with the detection of High Frequency QPO (HFQPO) in the power density spectra (PDS). The HFQPO is seen to vary in the frequency range of 67.96−70.62 Hz with percentage rms ∼0.83−1.90 % and significance varying from 1.63−7.75. The energy dependent power spectra show that the HFQPO features are dominant only in 6−25 keV energy band. The broadband energy spectra (0.7−50 keV) of SXT (Soft X-ray Telescope) and LAXPC (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 broadband spectral modelling indicates that the source spectra are well described by thermal Comptonization with electron temperature (kTe) of 2.07−2.43 keV and photon-index (Γnth) between 1.73−2.45 with an additional powerlaw component of photon-index (ΓPL) between 2.94−3.28. 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 M˙Edd. Moreover, we find the mass and spin of the source as 12.44−13.09 M⊙ and 0.990−0.997 with 90% confidence suggesting that GRS 1915+105 is a maximally rotating stellar mass X-ray binary black hole source.

Malu et al. (2020). Coronal Vertical Structure Variations in Normal Branch of GX 17+2 : AstroSaťs SXT and LAXPC perspective. arXiv:2009.11002 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 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 lightcurves and they exhibited correlated and anti-correlated lags of the order of a hundred seconds. Power density spectrum displayed NB oscillations of 6.7--7.8 Hz (quality factor 1.5--4.0). Spectral modeling resulted in inner disk radius of ∼ 12--16 km with Γ ∼ 2.31--2.44 indicating that disk is close to the ISCO and a similar value of disk 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 = vcorona/vdisk) 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 disk's inner region, it's 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 disk front strongly indicates that the varying corona structure is causing the X-ray variation in the NB of Z source GX 17+2.

Sudip Chakraborty et al. (2020). A spectral study of the black hole X-ray binary MAXI J1820+070 with AstroSat and NuSTAR. arXiv:2009.02465 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 broadband 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 March 2018. Implementing a combination of multi-colour 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-fit model suggests a low temperature disc (kTin∼0.3 keV), iron overabundance (AFe∼4−5 solar), a short lamp-post corona height (h≲8Rg), and a high corona temperature (kTe∼115−150 keV). Addition of a second Comptonisation component leads to a significantly better fit, with the kTe of the second Comptonisation 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, utilising the broader energy coverage of AstroSat, we estimate the black hole mass to be 6.7−13.9 M⊙, consistent with independent measurements reported in the literature.

Mudambi et al. (2020). Estimation of the black hole spin in LMC X-1 using AstroSat. MNRAS 498, 4404 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. Unlikemany other BHXBs, the black hole mass, source distance and binary orbital inclinationare well established. In this work, we report the results of simultaneous broadbandspectral studies of LMC X-1 carried out using the data from Soft X-ray Telescopeand Large Area X-ray Proportional Counter aboardAstroSatas observed on 2016November 26thand 2017 August 28th. The combined spectrum was modelled witha multicolour blackbody emission (diskbb), aGaussianalong with a Comptonizationcomponent (simpl) in the energy range 0.7−30.0 keV. The spectral analysis revealedthat the source was in its high soft state (Γ= 2.67+0.24−0.24andΓ= 2.12+0.19−0.20) with ahot disc (kTin= 0.86+0.01−0.01and kTin= 0.87+0.02−0.02). Thermal disc emission was fit witha relativistic model (kerrbb) and spin of the black hole was estimated to be 0.93+0.01−0.01and 0.93+0.04−0.03(statistical errors) for the twoEpochsthrough X-ray continuum-fitting,which agrees with the previous results.

Jaiswal et al. (2020). Revisiting the spectral and timing properties of 4U 1909+07 with NuSTAR and Astrosat. MNRAS 498, 4830 We present the results obtained from the analysis of high mass X-ray binary pulsar 4U 1909+07 using NuSTAR and Astrosat observations in 2015 and 2017 July, 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 years. 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 broadband energy spectrum of the pulsar is approximated by an absorbed high energy exponential cutoff 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 by 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.

Devasia et al. (2021). Thermonuclear X-ray bursts detected in Cyg X-2 using AstroSat/LAXPC. NewA 83, 101479 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 ∼2 s, and with fluences ∼1×10−8 ergs/cm2, 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.

Garg et al. (2020). Identifying the radiative components responsible for Quasi-Periodic Oscillations of black hole systems. MNRAS 498, 2757 While the dynamical origin of the variability observed in Galactic Black hole systems, such as quasi-periodic oscillations (QPOs), are 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 fit time-averaged spectral components reproduce the observed energy dependent fractional r.m.s 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 disk and a 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 disk, the truncated disk 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.

Bogensberger et al.(2020). An underlying clock in the extreme flip-flop state transitions of the black hole transient Swift J1658.2-4242. A&A 641, 101 Flip-flops are top-hat-like X-ray flux variations which occur 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 disk emission starts to dominate over coronal emission. Flip-flops have only rarely been observed and are poorly understood. Methods: We detect 15 flip-flops in the 2018 outburst of Swift J1658.2-4242, in observations by XMM-Newton, NuSTAR, Astrosat, Swift, Insight-HXMT, INTEGRAL, and ATCA. We analyse their light curves, search for periodicities, compute their PDS, and fit 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 ~100s, much larger than has been seen so far. We observe 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 to the PDS are delayed, but more rapid than changes in the light curve. Flip-flops occur in two intervals, separated by 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 2.761ks in the first interval, and 2.61ks in the second. Spectral analysis reveals the high and low flux flip-flop states to be similar, but distinct from intervals lacking flip-flops. A change in the inner temperature of the accretion disk is responsible for most of the flux difference in the flip-flops. We highlight the importance of correcting for the influence of the dust scattering halo on the X-ray spectra.

Bhulla et al.(2020). Detection of the thermonuclear X-ray bursts and dips from the X-ray binary 4U 1323-62 with AstroSat/LAXPC. RAA 20, 98 Using data from the Large Area X-ray Proportional Counter (LAXPC) on the \textit{AstroSat} satellite, we observed the Type-1 thermonuclear X-ray bursts from the Low Mass X-ray Binary Neutron Star 4U 1323-62. The observations of 4U 1323-62 were carried out during the performance verification phase of the \textit{AstroSat} satellite showed 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 seconds. Moreover, the light curve of 4U 1323-62 revealed the presence of two dips. We presented 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 ∼ 1.8 × 10−9 erg cm2 s−1. The radius of the blackbody is found to be highly consistent with the blackbody temperature and the blackbody flux of the bursts.

Agrawal and Nandi (2020). AstroSat view of LMC X-2: Evolution of broadband X-ray spectral properties along a complete Z-track. MNRAS 497, 3726 In this paper, we report the first results of the extragalactic Z-source LMC X-2 obtained using the ∼ 140 ks observations with {\it Large Area X-ray Proportional Counter (LAXPC)} and {\it Soft X-ray Telescope (SXT)} onboard {\it AstroSat}. The HID created with the {\it LAXPC} data revealed a complete Z-pattern of the source, showing all the three branches. We studied the evolution of the broadband 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 (kTe) 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 QPO (2.8~σ) in the FB. The intrinsic luminosity of the source varied between (1.03−1.79) × 1038 ergs/s. We discuss our results by comparing with other Z-sources and the previous observations of LMC X-2.

Bala et al. (2020). Time Evolution Of Cyclotron Line of Her X-1; A DetailedStatistical Analysis Including New ASTROSAT Data. MNRAS 497, 1029 The cyclotron line feature in the X-ray spectrum of the accretion powered pulsar HerX-1 has been observed and monitored for over three decades. The line energy exhibiteda slow secular decline over the period 1995-2014, with a possible (not confirmed) indi-cation of a reversal thereafter. Recent works have shown that the temporal evolutionof the line energy may be modelled as a flattening after an earlier decrease until MJD55400 (±200). In this work, we present the results of ASTROSAT observations in thecontext of earlier data and offer a common interpretation through a detailed study oftemporal and flux dependence. We find that the variation of the line energy does notsupport an upward trend but is consistent with the reported flattening after an earlierdecrease until MJD54487+515−469.

Blessy et al. (2020). AstroSat and MAXI view of the Black Hole binary 4U 1630-472 during 2016 and 2018 Outbursts. MNRAS 497, 1197 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 \textit{AstroSat} and \textit{MAXI}. The extensive coverage of the outbursts with \textit{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 broadband energy spectra (0.7−20.0 keV) of \textit{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 (Tin) ∼ 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 \textit{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 (QPOs) 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⊙. Finally, we discuss the possible implications of our findings.

Kallol Mukerjee et al. (2020). AstroSat observations of GRO J2058+42 during the 2019 outburst. ApJ 897, 73 We present results from AstroSat observation of the recent outburst of GRO J2058+42, an X-ray pulsar in a Be-binary system. The source was observed on April 10, 2019 by LAXPC and SXT instruments on AstroSat during its declining phase of the latest giant outburst. Light curves showed a strong pulsation of the pulsar with a period of 194.2201 \pm 0.0016 s, and a spin-up rate of (1.65\pm0.06)\times10^{-11} Hz s^{-1}. Intermittent flaring was detected in light curves between 3--80 keV energy band with increase in intensity by up to 1.8 times its average intensity. Pulse profiles obtained between 3--80 keV energy band of the pulsar showed strong dependence on energy. A broad peak was observed in the power density spectrum of the source consistently during AstroSat observations with its 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 observation 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 phase dependent variation in its energy and relative strength of these features. The spectrum was well fitted with an absorbed black-body, a Fermi Dirac cut-off model and alternatively with an absorbed CompTT model. Both these models were combined with a Fe-line and three Gaussian absorption lines to account for observed cyclotron resonance scattering features in the spectrum.

Amar Deo Chandra et al. (2020). Study of Recent outburst in the Be/X-ray binary RX J0209.6-7427withAstroSat: A new ultraluminous X-ray pulsar in the MagellanicBridge? MNRAS 495, 2664 We present the timing and spectral studies of RX J0209.6-7427 during its rare 2019 outburstusing observations with the Soft X-ray Telescope (SXT) and Large Area X-ray ProportionalCounter (LAXPC) instruments on theAstroSatsatellite. Pulsations having a periodicity of 9.29s were detected for the first time by theNICERmission in the 0.2-10 keV energy band and, asreported here, byAstroSatover a broad energy band covering 0.3-80 keV. The pulsar exhibitsa rapid spin-up during the outburst. Energy resolved foldedpulse profiles are generated inseveral energy bands in 3-80 keV. To the best of our knowledgethis is the first report of thetiming and spectral characteristics of this Be binary pulsar in hard X-rays. There is suggestionof evolution of the pulse profile with energy. The energy spectrum of the pulsar is determinedand from the best fit spectral values, the X-ray luminosity ofRX J0209.6-7427 is inferred tobe 1.6×1039ergs s−1. Our timing and spectral studies suggest that this source has features ofan ultraluminous X-ray pulsar in the Magellanic Bridge. Details of the results are presentedand discussed in terms of the current ideas.

Jayshree Roy et al. (2020). AstroSat observation of the Be/X-ray binary Pulsar 3A 0726-260(4U 0728-25). RAA 20, 155 Results on timing and spectral properties of the Be/X-ray binary pulsar 3A0726-260 (4U 0728-25) are presented. The binary was observed on 2016 May 6-7 withthe 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±0.3×10−11ergs cm−2sec−1in 0.4-20keV. Strong X-ray pulsations with a period of 103.144±0.001 seconds are detected in0.3-7 keV with the SXT and in 3-40 keV with the LAXPC. The pulseprofile is energydependent, and there is an indication that the pulse shape changes from a broad singlepulse to a double pulse at higher energy. At energies above 20keV, we report the first timedetection of pulsation period 103.145±0.001 seconds and the double peaked pulse profilefrom the source. The energy spectrum of the source is derivedfrom the combined analysisof the SXT and LAXPC spectral data in 0.4-20 keV. The best spectral fit is obtained bya power law model with a photon index (1.7±0.03) with high energy spectral cut-off at12.9±0.7 keV. A broad Iron line at∼6.3 keV is detected in the energy spectrum. Webriefly discuss the implications of these results.

Agrawal et al. (2020). AstroSat/LAXPC view of GX 17+2: Spectral Evolution along the Z-track. Ap&SS 365, 56B 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 {\it 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. A normal branch oscillation (NBO) with a centroid frequency 7.42±0.23 Hz, quality factor (Q) ∼ 4.88, rms 1.41±0.29\% and significance 5.1σ is detected at the middle of the NB. The parameters of the Comptonized emission show a systematic evolution along the Z-diagram. The optical depth of the corona increases as the source moves up along the FB, suggesting possible trigger of an outflow or dumping of the disc material in to the corona by radiation pressure.

Misra et al. (2020). Identification of QPO frequency of GRS 1915+105 as the relativistic dynamic frequency of a truncated accretion disk. ApJ 889, L36 We have analyzed AstroSat observations of the galactic micro-quasar 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 broad band spectra (1-50 keV) obtained from simultaneous LAXPC and SXT 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) as predicted decades ago by the relativistic standard accretion disk theory for a black hole with spin parameter of ~0.9. We show that this identification has been possible due to the simultaneous broad band spectral coverage with temporal information as obtained from AstroSat.

Sharma et al. (2020).A broadband look of the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 using AstroSat and XMM-Newton. MNRAS 492, 4361 SAX J1748.9-2021 is a transient accretion powered millisecond X-ray pulsar lo-cated in the Globular cluster NGC 6440. We report on the spectral and timing analy-sis of SAX J1748.9-2021 performed onAstroSatdata taken during its faint and shortoutburst of 2017. We derived the best-fitting orbital solution for the 2017 outburstand obtained an average local spin frequency of 442.361098(3) Hz. The pulse profileobtained from 3–7 keV and 7–20 keV energy bands suggest constant fractional am-plitude∼0.5% for fundamental component, contrary to previously observed energypulse profile dependence. OurAstroSatobservations revealed the source to be in a hardspectral state. The 1–50 keV spectrum from SXT and LAXPC on-boardAstroSatcanbe well described with a single temperature blackbody and thermal Comptonization.Moreover, we found that the combined spectra fromXMM-Newton(EPIC-PN) andAstroSat(SXT+LAXPC) indicated the presence of reflection features in the form ofiron (Fe Kα) line that we modeled with the reflection modelxillvercp. One of thetwo X-ray burst observed during theAstroSat/LAXPC observation showed hard X-ray emission (>30 keV) due to Compton up-scattering of thermal photons by the hotcorona. Time resolved analysis performed on the bursts revealed complex evolution inemission radius of blackbody for second burst suggestive of mild photospheric radiusexpansion.

Mudambi et al. (2020). Unveiling the temporal properties of MAXI J1820+070 through AstroSat observations. ApJ 889, L17 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 SXT and LAXPC on-board AstroSat.The observed combined spectra in the energy range 0.7-80 keV were well modeled using disk black-body emission, thermal Comptonization and a reflection component. The spectral analysis revealed that the source was in its hard spectral state (Gamma=1.61) with a cool disk (kTin=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.

Goswami et al. (2019). Unravelling the unusually curved X-ray spectrum of RGB J0710+591 using AstroSat observations. MNRAS 492, 796 We report the analysis of simultaneous multi-wavelength data of thehigh en-ergy peaked blazar RGB J0710+591 from the LAXPC, SXT and UVIT instrumentson-boardAstroSat. The wide band X-ray spectrum (0.35 – 30 keV) is modelled assynchrotron emission from a non-thermal distribution of high energy electrons. Thespectrum is unusually curved, with a curvature parameterβp∼6.4for a log parabolaparticle distribution, or a high energy spectral indexp2>4.5for a broken power-lawdistribution. The spectrum shows more curvature than an earlier quasi-simultaneousanalysis ofSwift-XRT/NuSTAR data where the parameters wereβp∼2.2orp2∼4.It has long been known that a power-law electron distribution can beproduced from aregion where particles are accelerated under Fermi process and the radiative losses inacceleration site decide the maximum attainable Lorentz factor,γmax. Consequently,this quantity decides the energy at which the spectrum curves steeply. We show thatsuch a distribution provides a more natural explanation for theAstroSatdata as wellas the earlier XRT/NuSTAR observation, making this as the first well constraineddetermination of the photon energy corresponding toγmax. This in turn provides anestimate of the acceleration time-scale as a function of magnetic field and Dopplerfactor. The UVIT observations are consistent with earlier optical/UV measurementsand reconfirm that they plausibly correspond to a different radiative component thanthe one responsible for the X-ray emission.

Jithesh et al., (2019). SPECTRAL AND TIMING PROPERTIES OF THE GALACTIC X-RAY TRANSIENT SWIFT J1658.2–4242 USING ASTROSATOBSERVATIONS. ApJ 887, 101 We present the X-ray timing and spectral analysis of the new Galactic X-ray transient Swift J1658.2-4242 observed with LAXPC and SXT instruments onboardAstrosat. We detect prominent C-typequasi-periodic oscillations (QPOs) of frequencies varying from∼1.5 Hz to∼6.6 Hz along withdistinct 2nd harmonics and sub-harmonics. The QPO detected at∼1.56 Hz drifts to a highercentroid frequency of∼1.74 in the course of the observation, while the QPO detected at∼6.6 Hzdisappeared during hard flarings. The fractional rms at the QPO and the sub-harmonic frequenciesincreases with photon energy, while at the 2nd harmonic frequencies the rms seems to be constant. Inaddition, we have observed soft time lag at QPO and sub-harmonic frequencies up to a time scale of∼35 ms, however, at the 2nd harmonic frequencies there is weak/zero time lag. We attempt spectralmodeling of the broadband data in the 0.7–25 keV band using the doubly absorbed disk plus thermalComptonization model. Based on the spectral and timing properties, we identified the source to bein the hard intermediate state of black hole X-ray binaries. To quantitatively fit the energy andfrequency-dependent fractional rms and time lag, we use a single zone fluctuation propagation modeland discuss our results in the context of that model.

Belloni et al., (2019). A variable-frequency HFQPO in GRS 1915+105 asobserved with Astrosat. MNRAS, 489, 1037 eFrom the analysis of more than 92 ks of data obtained with the laxpc instrumenton boardAstrosatwe have detected a clear high-frequency QPO whose frequencyvaries between 67.4 and 72.3 Hz. In the classification of variability classes of GRS1915+105, at the start of the observation period the source was in classωand at theend the variability was that of classμ: both classes are characterized by the absence ofhard intervals and correspond to disk-dominated spectra. After normalization to takeinto 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 HFQPOvariability at high energies lags that at lower energies and detect systematic variationswith 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 otherproperties of the source. We discuss the results in the framework of existing models,although the small (7%) variability observed is too small to draw firm conclusions.

Xiao et al., (2019). Timing analysis of Swift J1658.2–4242’s outburst in 2018 with Insight -HXMT, NICER and AstroSat. JHEAp 24, 30 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 analysed 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 prop- erties 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 under 4 Hz 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.

Yash Bhargava et al., (2019). Spectro-timing analysis of MAXI J1535-571 using AstroSat. MNRAS 488, 720 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 days 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 %. We modelled the spectra as a combination of radiation from a thermal disk component and a power-law. The timing analysis revealed the presence of strong Quasi Periodic Oscillations with centroid frequency νQPO fluctuating in the range 1.7-3.0 Hz. We found a tight correlation between the QPO centroid frequency νQPO and the power-law spectral index Γ, while νQPO appeared not to be correlated with the linearly-increasing flux itself. We discuss the implications of these results on physical models of accretion.

Varun Bahal et al., (2019). Pulse Phase Variation of Cyclotron Line in HMXB 4U 1907+09 with ASTROSAT LAXPC. ApJ 880, 61 We present timing and spectral analysis of data from an observation of the High Mass X-ray Binary pulsar 4U 1907+09 with the LAXPC 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 upto 40 keV and 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 ten 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.

Navin Sridhar et al., (2019). Broadband reflection spectroscopy of MAXI J1535-571 using AstroSat: Estimation of black hole mass and spin. MNRAS 487, 4221 We report the results from \textit{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 \textit{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 broadband (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 broadband spectral analysis. By fitting the reflection spectrum with the latest version of the \textsc{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 (\textsc{relxill}) and a general relativistic thin disc component (\texttt{Kerrbb}), and estimate the black hole's mass and distance to be 10.39+0.61−0.62M⊙ and 5.4+1.8−1.1 kpc respectively.

Sreehari et al., (2019). AstroSat view of MAXI J1535−571: broadband spectro-temporal features. MNRAS 487, 928 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 Hz 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 broadband 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 between 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_Edd assuming the source to be at a distance of 8 kpc and hosting a black hole with a mass of 6 M⊙. The physical model based on the two-component accretion flow gives disc accretion rates as high as ~ 1 m˙Edd and halo rate ~ 0.2 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 to 7.83 M⊙.

Maqbool et al., (2019). A stochastic propagation model to the energy dependent rapid temporal behaviour of Cygnus X-1 as observed by AstroSat in the hard state. MNRAS 486, 2964 We report the results from analysis of six observations of Cygnus X-1 by Large Area X-ray Proportional Counters (LAXPC) and Soft X-ray Telescope (SXT) on-board 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 disk 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 disk with a hot inner region. Unlike other propagation models, the hard X-ray emission (> 4 keV) is assumed to be from the hot inner disk by a single temperature thermal Comptonization process. The fluctuations first cause a variation in the temperature of the truncated disk and then the temperature of the inner disk after a frequency dependent time delay. We find that the model can explain the energy dependent rms and time-lag at different frequencies.

Bhulla et al., (2019). AstroSat observation of GX 5-1: Spectral and timing evolution. RAA 19, 114 We report on the first analysis of AstroSat observation of the Z-source GX 5- 1 on February 26-27, 2017. 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 well described by a disk emission and a thermal Comptonized component. The ratio of the disk flux to the total i.e. the disk flux ratio increases monotonically along the horizontal 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_{in}^{5.5} and not as T_{in}{4} suggesting that either the inner radii changes dramatically or that the disk is irradiated by the thermal component changing its hardness factor. The power spectra reveal a Quasi Periodic Oscillation whose frequency changes from \sim 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 \prop E0.8), while the harder X-ray seems to lag the soft ones with a time-delay of a 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.

Jayashree Roy et al., (2019). LAXPC / AstroSat Study of ~1 and ~2 mHz Quasi-periodic Oscillations in the Be/X-ray Binary 4U 0115+63 During its 2015 Outburst. ApJ, 872, 33 The Be X-ray Binary 4U 0115+63 was observed by 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 mHz 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 (RXTE)/Proportional Counter Array (PCA) data during 2001-11 also show presence of mHz QPOs during some of the outbursts and details of these QPOs are also reported. Possible models to explain the origin of the mHz 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.

Rawat et al., (2019). Study of timing evolution from non-variable to structured large-amplitude variability transition in GRS 1915+105 using AstroSat. ApJ 870, 4 In this work, we present a ∼90 ks continuous monitoring of the Galactic micro-quasar GRS 1915+105 with AstroSat when the source undergoes a major transition from a non-variable, χ class (similar to radio-quiet χ class) to a structured, large amplitude, periodic heartbeat state (similar to ρ class). We show that such 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 a 100-150 sec regular, structured, nearly periodic flares. The properties of a strong low-frequency quasi-periodic oscillations (LF 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 few important differences between the heartbeat state and the ρ class. Interestingly, the time-averaged LF QPO properties in the hard X-ray band is relatively stable in three states when compared to the significant evolution observed in the slow variability properties at mHz frequencies. Such relative stability of LF QPOs implies the inner disk-corona coupled accretion flow which determines the LF QPO properties, may be uninterrupted by the launch of long, large-amplitude flares.

Varun et al., (2019). Probing the Cyclotron line characteristics of 4U 1538–522 using AstroSat-LAXPC. MNRAS 484, L1 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 which spanned about one day 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 detection 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% 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.

Beri et al., (2019).Thermonuclear X-ray bursts in rapid succession in 4U 1636-536 with AstroSat-LAXPC MNRAS 482, 4397 MNRAS 482, 4397 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 minutes between 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 (QPO) at ~ 5 Hz. However, we did not find any evidence of kilo-hertz QPOs and/or X-ray burst oscillations, perhaps due to the hard spectral state of the source during this observation.

Pahari et al., (2018). AstroSat and Chandra view of the high soft state of 4U 1630-47 (4U 1630-472): evidence of the disk wind and a rapidly spinning black hole. ApJ 867,86 We present the X-ray spectral and timing analysis of the transient black hole X-ray binary 4U 1630– 47, observed with the AstroSat, Chandra and MAXI space missions during its soft X-ray outburst in 2016. The outburst, from the rising phase until the peak, is neither detected in hard X-rays (15-50 keV) by the Swift/BAT nor in radio. Such non-detection along with the source behavior in the hardness- intensity and color-color diagrams obtained using MAXI data confirm that both Chandra and AstroSat observations were performed during the high soft spectral state. The High Energy Grating (HEG) spectrum from the Chandra high-energy transmission grating spectrometer (HETGS) shows two strong, +0.002 +0.004 moderately blueshifted absorption lines at 6.705−0.002 keV and 6.974−0.003 keV, which are produced by Fe XXV and Fe XXVI in a low-velocity ionized disk wind. The corresponding outflow velocity is determined to be 366±56 km/s. Separate spectral fits of Chandra/HEG, AstroSat/SXT+LAXPC and Chandra/HEG+AstroSat/SXT+LAXPC data show that the broadband continuum can be well described with a relativistic disk-blackbody model, with the disk flux fraction of ∼ 0.97. Based on the best-fit continuum spectral modeling of Chandra, AstroSat and Chandra+AstroSat joint spectra and using the Markov Chain Monte Carlo simulations, we constrain the spectral hardening factor at 1.56−0.06 +0.14 and the dimensionless black hole spin parameter at 0.92 ± 0.04 within the 99.7% confidence interval. Our conclusion of a rapidly-spinning black hole in 4U 1630–47 using the continuum spectrum method is in agreement with a previous finding applying the reflection spectral fitting method.

Sudip Bhattacharyya et al., (2018). Effects of thermonuclear X-ray bursts on non-burst emissions in the soft state of 4U 1728--34. ApJ 860, 88 It has recently been shown that the persistent emission of a neutron star low-mass X-ray binary (LMXB) evolves during a thermonuclear (type-I) X-ray burst. The reason of this evolution, however, is not securely known. This uncertainty can introduce significant systematics in the neutron star radius measurement using burst spectra, particularly if an unknown but significant fraction of the burst emission, which is reprocessed, contributes to the changes in the persistent emission during the burst. Here, by analyzing individual burst data of AstroSat/LAXPC from the neutron star LMXB 4U 1728--34 in the soft state, we show that the burst emission is not significantly reprocessed by a corona covering the neutron star. Rather, our analysis suggests that the burst emission enhances the accretion disk emission, possibly by increasing the accretion rate via disk. This enhanced disk emission, which is Comptonized by a corona covering the disk, can explain an increased persistent emission observed during the burst. This finding provides an understanding of persistent emission components, and their interaction with the thermonuclear burst emission. Furthermore, since burst photons are not significantly reprocessed, non-burst and burst emissions can be reliably separated, which is required to reduce systematic uncertainties in the stellar radius measurement.

V. K. Agrawal et al., (2018). Spectral and Timing Properties of Atoll Source 4U 1705-44 : LAXPC/AstroSat Results MNRAS 477, 5437 In this paper, we present the first results of spectral and timing properties of the atoll source 4U 1705-44 using ∼ 100 AstroSat. ks data obtained with Large Area X-ray Proportional Counter (LAXPC) onboard The source was in the high-soft state during our observations and traced out a banana track in the Hardness Intensity Diagram (HID). We study the evolution of the Power Density Spectra (PDS) and the energy spectra along the HID. PDS show presence of a broad Lorentzian feature (Peaked Noise or PN) centered at 1 − 13 Hz and a very low frequency noise (VLFN). The energy spectra can be described by sum of a thermal Comptonized component, a power-law and a broad iron line. The hard tail seen in the energy spectra is variable and contribute 4 − 30% of the total flux. The iron line seen in this source is broad (FWHM ∼ 2 keV) and strong (EW ∼ 369 − 512 eV). Only relativistic smearing in the accretion disc can not explain the origin of this feature and requires other mechanism such as broadening by Comptonization process in the external part of the ‘Comptonized Corona’. A subtle and systematic evolution of the spectral parameters (optical depth, electron temperature etc.) is seen as the source moves along the HID. We study the correlation between frequency of the PN and the spectral parameters. PN frequency seems to be correlated with the strength of the corona. We discuss the implication of the results in the paper.

Pahari et al., (2018). Extensive Broadband X-Ray Monitoring During the Formation of a Giant Radio Jet Base in Cyg X-3 with AstroSat. ApJ 853, L11 We present X-ray spectral and timing behavior of Cyg X-3 as observed by AstroSat during the onset of a giant radio flare on 2017 April 1–2. Within a timescale of a few hours, the source shows a transition from the hypersoft state (HPS) to a more luminous state (we termed as the very high state), which coincides with the time of the steep rise in radio flux density by an order of magnitude. Modeling the Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) spectra jointly in 0.5–70.0 keV, we found that the first few hours of the observation is dominated by the HPS with no significant counts above 17 keV. Later, an additional flat power-law component suddenly appeared in the spectra that extends to very high energies with the power-law photon index of {1.49}-0.03+0.04. Such a flat power-law component has never been reported from Cyg X-3. Interestingly the fitted power-law model in 25–70 keV, when extrapolated to the radio frequency, predicts the radio flux density to be consistent with the trend measured from the RATAN-600 telescope at 11.2 GHz. This provides direct evidence of the synchrotron origin of flat X-ray power-law component and the most extensive monitoring of the broadband X-ray behavior at the moment of decoupling the giant radio jet base from the compact object in Cyg X-3. Using SXT and LAXPC observations, we determine the giant flare ejection time as MJD 57845.34 ± 0.08 when 11.2 GHz radio flux density increases from ∼100 to ∼478 mJy.

Pahari et al., (2017). X-Ray Timing Analysis of Cyg X-3 Using AstroSat/LAXPC: Detection of Milli-hertz Quasi-periodic Oscillations during the Flaring Hard X-Ray State. ApJ 849, 16 We present here results from the X-ray timing and spectral analysis of the X-ray binary Cyg X-3 using observations from the Large Area X-ray proportional Counter on board AstroSat. Consecutive light curves observed over a period of one year show the binary orbital period of 17253.56 ± 0.19 s. Another low-amplitude, slow periodicity of the order of 35.8 ± 1.4 days is observed, which may be due to the orbital precession as suggested earlier by Molteni et al. During the rising binary phase, power density spectra from different observations during the flaring hard X-ray state show quasi-periodic oscillations (QPOs) at ˜5-8 mHz, ˜12-14 mHz, and ˜18-24 mHz frequencies at the minimum confidence of 99%. However, during the consecutive binary decay phase, no QPO is detected up to 2σ significance. Energy-dependent time-lag spectra show soft lag (soft photons lag hard photons) at the mHz QPO frequency and the fractional rms of the QPO increases with the photon energy. During the binary motion, the observation of mHz QPOs during the rising phase of the flaring hard state may be linked to the increase in the supply of the accreting material in the disk and corona via stellar wind from the companion star. During the decay phase, the compact source moves in the outer wind region causing the decrease in supply of material for accretion. This may cause weakening of the mHz QPOs below the detection limit. This is also consistent with the preliminary analysis of the orbital phase-resolved energy spectra presented in this paper.

Antia et al., (2017). Calibration of the Large Area X-Ray Proportional Counter (LAXPC) Instrument on board AstroSat. ApJS 231, 10 We present the calibration and background model for the Large Area X-ray Proportional Counter (LAXPC) detectors on board AstroSat. The LAXPC instrument has three nominally identical detectors to achieve a large collecting area. These detectors are independent of each other, and in the event analysis mode they record the arrival time and energy of each photon that is detected. The detectors have a time resolution of 10 μs and a dead-time of about 42 μs. This makes LAXPC ideal for timing studies. The energy resolution and peak channel-to-energy mapping were obtained from calibration on the ground using radioactive sources coupled with GEANT4 simulations of the detectors. The response matrix was further refined from observations of the Crab after launch. At around 20 keV the energy resolution of the detectors is 10%-15%, while the combined effective area of the three detectors is about 6000 cm2.

Agrawal et al., (2017). Large Area X-Ray Proportional Counter (LAXPC) Instrument on AstroSat and Some Preliminary Results from its Performance in the Orbit. JApA 38, 30 Large area X-ray propositional counter (LAXPC) instrument on AstroSat is aimed at providing high time resolution X-ray observations in 3-80 keV energy band with moderate energy resolution. To achieve large collecting area, a cluster of three co-aligned identical LAXPC detectors, is used to realize an effective area in access of {˜ }6000 cm2 at 15 keV. The large detection volume of the LAXPC detectors, filled with xenon gas at {˜ }2 atmosphere pressure, results in detection efficiency greater than 50%, above 30 keV. In this article, we present salient features of the LAXPC detectors, their testing and characterization in the laboratory prior to launch and calibration in the orbit. Some preliminary results on timing and spectral characteristics of a few X-ray binaries and other type of sources, are briefly discussed to demonstrate that the LAXPC instrument is performing as planned in the orbit.

Yadav et al., (2017). Large Area X-ray Proportional Counter (LAXPC) Instrument on AstroSat. Current Science, 113, 591 Large Area X-ray Proportional Counter (LAXPC) is one of the major AstroSat payloads. LAXPC instrument will provide high time resolution X-ray observations in 3 to 80 keV energy band with moderate energy resolution. A cluster of three co-aligned identical LAXPC detectors is used in AstroSat to provide large collection area of more than 6000 cm2 . The large detection volume (15 cm depth) filled with xenon gas at about 2 atmosphere pressure, results in detection efficiency greater than 50%, above 30 keV. With its broad energy range and fine time resolution (10 microsecond), LAXPC instrument is well suited for timing and spectral studies of a wide variety of known and transient X-ray sources in the sky. We have done extensive calibration of all LAXPC detectors using radioactive sources as well as GEANT4 simulation of LAXPC detectors. We describe in brief some of the results obtained during the payload verification phase along with LXAPC capabilities.

Verdhan et al., (2017). AstroSat/LAXPC Detection of Millisecond Phenomena in 4U 1728-34. ApJ 841, 41 The low-mass X-ray binary 4U 1728-24 was observed with AstroSat/LAXPC on 2016 March 8th. Data from a randomly chosen orbit of over 3 ks was analyzed for detection of rapid intensity variations. We found that the source intensity was nearly steady but, toward the end of the observation, a typical Type-1 burst was detected. Dynamical power spectrum of the data in the 3-20 keV band, reveals the presence of a kHz Quasi-Periodic Oscillation (QPO) for which the frequency drifted from ˜815 Hz at the beginning of the observation to about 850 Hz just before the burst. The QPO is also detected in the 10-20 keV band, which was not obtainable by earlier RXTE observations of this source. Even for such a short observation with a drifting QPO frequency, the time lag between the 5-10 and 10-20 keV bands can be constrained to be less than 100 microseconds. The Type-1 burst that lasted for about 20 s had a typical profile. During the first four seconds, dynamic power spectra reveal a burst oscillation for which the frequency increased from ˜361.5 to ˜363.5 Hz. This is consistent with the earlier results obtained with RXTE/PCA, showing the same spin frequency of the neutron star. The present results demonstrate the capability of the LAXPC instrument for detecting millisecond variability even from short observations. After RXTE ceased operation, LAXPC on AstroSat is the only instrument at present with the capability of detecting kHz QPOs and other kinds of rapid variations from 3 keV to 20 keV and possibly at higher energies as well.

Misra et al., (2017). AstroSat/LAXPC Observation of Cygnus X-1 in the Hard State. ApJ 835, 195 We report the first analysis of data from AstroSat/LAXPC observations of Cygnus X-1 in 2016 January. LAXPC spectra reveals that the source was in the canonical hard state, represented by a prominent thermal Comptonization component having a photon index of ˜1.8 and high temperature of kTe > 60 keV along with weak reflection and possible disk emission. The power spectrum can be characterized by two broad lorentzian functions centered at ˜0.4 and ˜3 Hz. The rms of the low-frequency component decreases from ˜15% at around 4 keV to ˜10% at around 50 keV, while that of the high-frequency one varies less rapidly from ˜13.5% to ˜11.5% in the same energy range. The time lag between the hard (20-40 keV) and soft (5-10 keV) bands varies in a step-like manner being nearly constant at ˜50 milliseconds from 0.3 to 0.9 Hz, decreasing to ˜8 milliseconds from 2 to 5 Hz and finally dropping to ˜2 milliseconds for higher frequencies. The time lags increase with energy for both the low and high-frequency components. The event mode LAXPC data allows for flux resolved spectral analysis on a timescale of 1 s, which clearly shows that the photon index increased from ˜1.72 to ˜1.80 as the flux increased by nearly a factor of two. We discuss the results in the framework of the fluctuation propagation model.

Yadav et al., (2016). Astrosat/LAXPC Reveals the High-energy Variability of GRS 1915+105 in the X Class. ApJ 833, 27 We present the first quick look analysis of data from nine AstroSat's Large Area X-ray Proportional Counter (LAXPC) observations of GRS 1915+105 during 2016 March when the source had the characteristics of being in the Radio-quiet χ class. We find that a simple empirical model of a disk blackbody emission, with Comptonization and a broad Gaussian Iron line can fit the time-averaged 3-80 keV spectrum with a systematic uncertainty of 1.5% and a background flux uncertainty of 4%. A simple dead time corrected Poisson noise level spectrum matches well with the observed high-frequency power spectra till 50 kHz and as expected the data show no significant high-frequency (\gt 20 {Hz}) features. Energy dependent power spectra reveal a strong low-frequency (2-8 Hz) quasi-periodic oscillation and its harmonic along with broadband noise. The QPO frequency changes rapidly with flux (nearly 4 Hz in ˜5 hr). With increasing QPO frequency, an excess noise component appears significantly in the high-energy regime (\gt 8 keV). At the QPO frequencies, the time-lag as a function of energy has a non-monotonic behavior such that the lags decrease with energy till about 15-20 keV and then increase for higher energies. These first-look results benchmark the performance of LAXPC at high energies and confirms that its data can be used for more sophisticated analysis such as flux or frequency-resolved spectro-timing studies.

Roy et al., (2016). Performance of large area x-ray proportional counters in a balloon experiment. ExA 42, 249 AstroSat is India's first satellite fully devoted to astronomical observations covering a wide spectral band from optical to hard X-rays by a complement of 4 co-aligned instruments and a Scanning Sky X-ray Monitor. One of the instruments is Large Area X-ray Proportional Counter with 3 identical detectors. In order to assess the performance of this instrument, a balloon experiment with two prototype Large Area X-ray Proportional Counters (LAXPC) was carried out on 2008 April 14. The design of these LAXPCs was similar to those on the ASTROSAT except that their field of view (FOV) was 3 ∘ × 3 ∘ versus FOV of 1 ∘ × 1 ∘ for the LAXPCs on the ASTROSAT. The LAXPCs are aimed at the timing and spectral studies of X-ray sources in 3-80 keV region. In the balloon experiment, the LAXPC, associated electronics and support systems were mounted on an oriented platform which could be pre-programmed to track any source in the sky. A brief description of the LAXPC design, laboratory tests, calibration and the detector characteristics is presented here. The details of the experiment and background counting rates of the 2 LAXPCs at the float altitude of about 41 km are presented in different energy bands. The bright black hole X-ray binary Cygnus X-1 (Cyg X-1) was observed in the experiment for ˜ 3 hours. Details of Cyg X-1 observations, count rates measured from it in different energy intervals and the intensity variations of Cyg X-1 detected during the observations are presented and briefly discussed.

Yadav et al. 2016, Large Area X-ray Proportional Counter (LAXPC) instrument onboard AstroSat. SPIE 9905, 1D AstroSat, India's first dedicated astronomy space mission was launched on September 28, 2015. The Large Area X-ray Proportional Counter (LAXPC) is one of the major payloads on ASTROSAT. A cluster of three co-aligned identical LAXPC detectors provide large area of collection .The large detection volume (15 cm depth) filled with mixture of xenon gas (90(%) and methane (10%) at 2 atmospheres pressure, results in detection efficiency greater than 50%, above 30 keV. The LAXPC instrument is best suited for X-ray timing and spectral studies. It will provide the largest effective area in 3-80 keV range among all the satellite missions flown so far worldwide and will remain so for the next 5-10 years. The LAXPC detectors have been calibrated using radioactive sources in the laboratory. GEANT4 simulation for LAXPC detectors was carried out to understand detector background and its response. The LAXPC instrument became fully operational on 19th October 2015 for the first time in space. We have performed detector calibration in orbit. The LAXPC instrument is functioning well and has achieved all detector parameters proposed initially. In this paper, we will describe LAXPC detector calibration in lab as well as in orbit along with first results.

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