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A comprehensive wideband spectral analysis of the brightest black hole X-ray binary 4U 1543−47 during its 2021 outburst is carried out for the first time using NICER, NuSTAR , and AstroSat observations by phenomenological and reflection modelling. The source attains a super-Eddington peak luminosity and remains in the soft state, with a small fraction (⁠|$\lt 3{{\ \rm per\ cent}}$|⁠) of the inverse-Comptonized photons. The spectral modelling reveals a steep photon index (Γ ∼ 2–2.6) and relatively high inner disc temperature (T in ∼ 0.9–1.27 keV). The line-of-sight column density varies between (0.45–0.54) × 1022 cm−2. Reflection modelling using the RELXILL model suggests that 4U 1543−47 is a low-inclination system (θ ∼ 32°–40°). The accretion disc is highly ionized (log ξ > 3) and has super solar abundance (3.6–10 A Fe,⊙) over the entire period of study. We detected a prominent dynamic absorption feature between ∼8 and 11 keV in the spectra throughout the outburst. This detection is the first of its kind for X-ray binaries. We infer that the absorption of the primary X-ray photons by the highly ionized, fast-moving disc winds can produce the observed absorption feature. The phenomenological spectral modelling also shows the presence of a neutral absorption feature ∼7.1–7.4 keV, and both ionized and neutral absorption components follow each other with a delay of a typical viscous time-scale of 10–15 d. [ABSTRACT FROM AUTHOR]

We present the results on broad-band X-ray properties of persistent black hole binaries GRS 1758−258 and 1E 1740.7−2942 using AstroSat, NuSTAR , and Swift – XRT observations carried out during 2016–2022. We perform spectral modelling of both sources after eliminating the contamination in their LAXPC spectra from nearby X-ray sources. Preliminary spectral modelling using Comptonization and line emission (∼6.4 keV) models suggest that GRS 1758−258 occupies both dim-soft state (kT bb = 0.37 ± 0.01 keV, Γ ∼ 5.9, |$L_{bol}=1{{\ \rm per\ cent}}$| of Eddington luminosity L Edd) and hard state (Γ = 1.64–2.22, kT e = 4–45 keV, L bol = 1–5 per cent L Edd) that requires a multicolour disc blackbody model (kT in = 0.54 ± 0.01 keV) occasionally. 1E 1740.7−2942 instead is found only in hard state (Γ = 1.67–2.32, kT e = 5–16 keV, L bol = 1–2 per cent L Edd). Reflection properties of both sources are studied by applying relativistic reflection model RELXILL to the broad-band spectra. Our results from AstroSat and NuSTAR consistently unveiled the presence of a Comptonizing region along with an ionized reflection region (ionization parameter log ξ = 2.7–3.8 and 2.7–4.7 erg cm s−1 in GRS 1758−258 and 1E 1740.7−2942, respectively) in both sources. Reflection modelling revealed GRS 1758−258 to have a high metal abundance (⁠|$A_{fe}=3.9^{+0.4}_{-0.3}$| times solar metal abundance) and inclination angle (i) of 61 ± 2°. In case of 1E 1740.7−2942, i is constrained to be 55 ± 1°. Finally, we discuss the implication of our findings in the context of accretion dynamics by comparing our results with the previous studies. [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 present a comprehensive study of GRS 1915+105 in wide-energy band (0.5–60 keV) using AstroSat – SXT and LAXPC observations during the period of 2016–2019. The MAXI X-ray light curve of the source shows rise and decay profiles similar to canonical outbursting black holes. However, the source does not follow the exemplary 'q'-diagram in the hardness–intensity diagram. Model independent analysis of light curves suggest that GRS 1915+105 displays various types of variability classes (δ, χ, ρ, κ, ω, and γ). We also report possible transitions from one class to another (χ → ρ, ρ → κ via an 'unknown' class and ω → γ → ω + γ) within a few hours duration. Detailed 'spectrotemporal' analysis indicates a gradual increase in the photon index (Γ) from 1.83 to 3.8, disc temperature (kT in) from 1.33 to 2.67 keV, and quasi-periodic oscillation frequency (ν) from 4 to 5.64 Hz during the rise, while the parameters decrease to Γ ∼1.18, kT in ∼1.18 keV, and ν ∼1.38 Hz, respectively, in the decline phase. The source shows maximum bolometric luminosity (L bol) during the peak at ∼36 per cent of Eddington luminosity (L Edd), and a minimum of ∼2.4 per cent of L Edd during the decay phase. Further evolution of the source towards an obscured low-luminosity (L bol of ∼1 per cent L Edd) phase, with a decrease in the intrinsic bolometric luminosity of the source due to obscuration, has also been indicated from our analysis. The implication of our results are discussed in the context of accretion disc dynamics around the black hole. [ABSTRACT FROM AUTHOR]

We study the spectral and temporal properties of MAXI J0637-430 during its 2019–2020 outburst using Neutron Star Interior Composition Explorer (NICER), AstroSat , and Swift–XRT data. The source was in a disc dominant state within a day of its detection and traces out a 'c' shaped profile in the HID, similar to the 'mini'-outbursts of the recurrent BHB 4U 1630-472. Energy spectrum is obtained in the 0.5−10 keV band with NICER and Swift–XRT , and 0.5−25 keV with AstroSat. The spectra can be modelled using a multicolour disc emission (DISKBB) convolved with a thermal Comptonization component (thcomp). The disc temperature decreases from 0.6 to 0.1 keV during the decay with a corresponding decrease in photon index (Γ) from 4.6 to 1.8. The fraction of Compton-scattered photons (f cov) remains <0.3 during the decay upto 2020 mid-January and gradually increases to 1 as the source reaches hard state. Power density spectra generated in the 0.01−100 Hz range display no quasi-periodic oscillations, although band-limited noise is seen towards the end of 2020 January. During AstroSat observations, Γ lies in the range 2.3−2.6 and rms increases from 11 to 20 per cent, suggesting that the source was in an intermediate state till 2019 November 21. Spectral fitting with the relativistic disc model (kerrbb), in conjunction with the soft-hard transition luminosity, favour a black hole with mass |$3\!-\!19\, \mathrm{ M}_{\odot }$| with retrograde spin at a distance <15 kpc. Finally, we discuss the possible implications of our findings. [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]

We study the properties of two-temperature accretion flow around a non-rotating black hole in presence of various dissipative processes where pseudo-Newtonian potential is adopted to mimic the effect of general relativity. The flow encounters energy loss by means of radiative processes acted on the electrons and at the same time, flow heats up as a consequence of viscous heating effective on ions. We assumed that the flow is exposed with the stochastic magnetic fields that leads to Synchrotron emission of electrons and these emissions are further strengthen by Compton scattering. We obtain the two-temperature global accretion solutions in terms of dissipation parameters, namely, viscosity (α) and accretion rate (m), and find for the first time in the literature that such solutions may contain standing shock waves. Solutions of this kind aremultitransonic in nature, as they simultaneously pass through both inner critical point (xin) and outer critical point (xout) before crossing the black hole horizon. We calculate the properties of shock-induced global accretion solutions in terms of the flow parameters.We further show that two-temperature shocked accretion flow is not a discrete solution, instead such solution exists for wide range of flow parameters.We identify the effective domain of the parameter space for standing shock and observe that parameter space shrinks as the dissipation is increased. Since the post-shock region is hotter due to the effect of shock compression, it naturally emits hard X-rays, and therefore, the two-temperature shocked accretion solution has the potential to explain the spectral properties of the black hole sources. [ABSTRACT FROM AUTHOR]

We investigate the global structure of the advection dominated accretion flow around a Schwarzschild black hole where the accretion disc is threaded by toroidal magnetic fields. We consider synchrotron radiative process as an effective cooling mechanism active in the flow. With this, we obtain the global transonic accretion solutions by exploring the variety of boundary conditions and dissipation parameters, namely accretion rate (m) and viscosity (αB). The fact that depending on the initial parameters, steady state accretion flows can possess centrifugally supported shock waves. These global shock solutions exist even when the level of dissipation is relatively high. We study the properties of shock waves and observe that the dynamics of the post-shock corona (hereafter, PSC) is regulated by the flow parameters. Interestingly, we find that shock solution disappears completely when the dissipation parameters exceed their critical values. We calculate the critical values of viscosity parameter (αBcri) adopting the canonical values of adiabatic indices as γ = 4/3 (ultrarelativistic) and 1.5 (seminon-relativistic) and find that in the gas pressure dominated domain, αBcri∼ 0.4 for γ = 4/3 and αBcri ∼ 0.27 for γ = 1.5, respectively.We further show that global shock solutions are relatively more luminous compared to the shock free solutions. Also, we have calculated the synchrotron spectra for shocked solutions. When the shock is considered to be dissipative in nature, it would have an important implication as the available energy at PSC can be utilized to power the outflowing matter escaped from PSC. Towards this, we calculate the maximum shock luminosity and discuss the observational implication of our present formalism. [ABSTRACT FROM AUTHOR]

We compute the effects of the centrifugal pressure supported shock waves on the emitted spectrum from an accretion disk primarily consisting of low angular momentum matter. Electrons are very efficiently accelerated by the accretion shock and acquire power-law distribution. The accelerated particles in turn emit synchrotron radiation in the presence of a stochastic magnetic field in equipartition with the gas. Efficient cooling of the electrons by these soft photons reduces its temperature in comparison to the protons. We explore the nature of the broadband spectra by using Comptonization, bremsstrahlung and synchrotron emission. We then show that there could be two crossing points in a broadband spectrum, one near ~10 keV and the other ~300–400 keV. [ABSTRACT FROM AUTHOR]