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A large number of unidentified sources found by astronomical surveys and other observations necessitate the use of an automated classification technique based on machine learning (ML) methods. The aim of this paper is to find a suitable automated classifier to identify the point X-ray sources in the Chandra Source Catalogue (CSC) 2.0 in the categories of active galactic nuclei (AGN), X-ray emitting stars, young stellar objects (YSOs), high-mass X-ray binaries (HMXBs), low-mass X-ray binaries (LMXBs), ultra luminous X-ray sources (ULXs), cataclysmic variables (CVs), and pulsars. The catalogue consists of ≈317 000 sources, out of which we select 277 069 point sources based on the quality flags available in CSC 2.0. In order to identify unknown sources of CSC 2.0, we use MW features, such as magnitudes in optical/ultraviolet bands from Gaia-EDR3, Sloan Digital Sky Survey, and GALEX, and magnitudes in infrared bands from 2MASS, WISE, and MIPS-Spitzer, in addition to X-ray features (flux and variability) from CSC 2.0. We find the Light Gradient Boosted Machine, an advanced decision tree-based ML classification algorithm, suitable for our purpose and achieve 93  per  cent precision, 93  per  cent recall score, and 0.91 Mathew's Correlation coefficient score. With the trained classifier, we identified 54 770 (14 066) sources with more than 3σ (4σ) confidence, out of which there are 32 600 (8574) AGNs, 16,148 (5,166) stars, 5,184 (208) YSOs, 439 (46) HMXBs, 197 (71) LMXBs, 50 (0) ULXs, 89 (1) CVs, and 63 (0) pulsars. This method can also be useful for identifying sources of other catalogues reliably. [ABSTRACT FROM AUTHOR]

We present a comprehensive spectral and temporal study of the black hole X-ray transient MAXI J1820+070 during its outbursts in 2018 using Swift/XRT, NICER, NuSTAR , and AstroSat observations. The Swift/XRT and NICER spectral study shows a plateau in the light curve with spectral softening (hardness changes from |$\sim \, 2.5$| to 2) followed by a gradual decline without spectral softening during the first outburst. Also, spectral modelling suggests that the first outburst is in the low/hard state throughout with a truncated disc, whereas the thermal disc emission dominates during the second outburst. During the entire outburst, strong reflection signature (reflection fraction varies in the range |$\sim \, 0.38\!-\!3.8$|⁠) is observed in the simultaneous wideband (NICER – NuSTAR, XRT – NuSTAR, AstroSat) data due to the presence of a dynamically evolving corona. The NICER timing analysis shows quasi-periodic oscillation signatures and the characteristic frequency increases (decreases) in the plateau (decline) phase with time during the first outburst. We understand that the reduction of the electron cooling time-scale in the corona due to spectral softening and the resonance oscillation with the local dynamical time-scale may explain the above behaviour of the source during the outburst. Also, we propose a possible scenario of outburst triggering and the associated accretion geometry of the source. [ABSTRACT FROM AUTHOR]

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 e) of 2.07–2.43 keV and photon index (Γnth) between 1.73 and 2.45 with an additional powerlaw component of photon index (ΓPL) 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⊙ 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. [ABSTRACT FROM AUTHOR]

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 in) ∼ 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 ⊙. Finally, we discuss the possible implications of our findings. [ABSTRACT FROM AUTHOR]

We study the two-temperature magnetized advective accretion flow around the Kerr black holes (BHs). During accretion, ions are heated up due to viscous dissipation, and when Coulomb coupling becomes effective, they transfer a part of their energy to the electrons. On the contrary, electrons lose energy due to various radiative cooling processes, namely bremsstrahlung, synchrotron, and Comtonization processes, respectively. To account for the magnetic contribution inside the disc, we consider the toroidal magnetic fields which are assumed to be dominant over other components. Moreover, we adopt the relativistic equation of state to describe the thermal characteristics of the flow. With this, we calculate the global transonic accretion solutions around the rotating BHs. We find that accretion solution containing multiple critical points may harbour shock wave provided the standing shock conditions are satisfied. Further, we investigate the shock properties, such as shock location (xs) and compression ratio (R) that delineate the post-shock corona (hereafter PSC) and find that the dynamics of PSC is controlled by the flow parameters, such as accretion rate (⁠|${\dot{m}}$|⁠) and magnetic fields (β, defined as the ratio of gas pressure to the magnetic pressure), etc. Finally, we calculate the emission spectra of the accretion flows containing PSC and indicate that both |${\dot{m}}$| and β play the pivotal roles in explaining the spectral state transitions commonly observed for BH X-ray binaries. [ABSTRACT FROM AUTHOR]

Abstract We carried out spectro-temporal analysis of the archived data from multiple outbursts spanning over the last two decades from the black hole X-ray binary GX 339-4. In this paper, the mass of the compact object in the X-ray binary system GX 339-4 is constrained based on three indirect methods. The first method uses broadband spectral modelling with a two component flow structure of the accretion around the black hole. The broadband data are obtained from RXTE (Rossi X-ray Timing Explorer) in the range 3.0 to 150.0 keV and from Swift and NuSTAR (Nuclear Spectroscopic Telescope Array) simultaneously in the range 0.5 to 79.0 keV. In the second method, we model the time evolution of Quasi-periodic Oscillation (QPO) frequencies, considering it to be the result of an oscillating shock that radially propagates towards or away from the compact object. The third method is based on scaling a mass dependent parameter from an empirical model of the photon index (Γ) – QPO (ν) correlation. We compare the results at 90 percent confidence from the three methods and summarize the mass estimate of the central object to be in the range 8.28 – 11.89 M ⊙ . [ABSTRACT FROM AUTHOR]