We study the spectral and temporal properties of MAXI J0637-430 during its 2019-2020 outburst using 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 Comptonisation component (thcomp). The disc temperature decreases from 0.6 keV 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 (fcov) remains < 0.3 during the decay upto mid-January 2020 and gradually increases to 1 as the source reaches hard state. Power Density Spectra (PDS) generated in the 0.01-100 Hz range display no Quasi-periodic Oscillations (QPOs) although band-limited noise (BLN) is seen towards the end of January 2020. During AstroSat observations, Γ lies in the range 2.3 − 2.6 and rms increases from 11 to 20%, suggesting that the source was in an intermediate state till 21 November 2019. Spectral fitting with the relativistic disc model (kerrbb), in conjunction with the soft-hard transition luminosity, favour a black hole with mass 3 − 19 M⊙ with retrograde spin at a distance <15 kpc. Finally, we discuss the possible implications of our findings.

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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 ($< 3\%$) of the inverse-Comptonized photons. The spectral modelling reveals a steep photon index (Γ ∼ 2 − 2.6) and relatively high inner disk temperature (Tin ∼ 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 disk is highly ionized (log ξ > 3) and has super solar abundance (3.6−10 AFe, ⊙) over the entire period of study. We detected a prominent dynamic absorption feature between ∼8 − 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 disk-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 timescale of 10 − 15 days.

We present the first results of extragalactic black hole (BH) X-ray binaries LMC X-1 and LMC X-3 using all the archival and legacy observations by AstroSat during the period of 2016–2020. Broad-band energy spectra (0.5–20 keV) of both sources obtained from the Soft X-ray Telescope and Large Area X-ray Proportional Counter on-board AstroSat are characterized by strong thermal disc blackbody component (kTin ∼ 1keV, $f_{\mathrm{ disc}}\gt 79{{\ \rm per\ cent}}$) along with a steep power-law (Γ ∼ 2.4–3.2). Bolometric luminosity of LMC X-1 varies from $7{\,\mathrm{ to}\,}10{{\ \rm per\ cent}}$ of Eddington luminosity (LEdd) and for LMC X-3 is in the range $7{-}13{{\ \rm per\ cent}}$ of LEdd. We study the long-term variation of light curve using MAXI data and find the fractional variance to be ${\sim}25{{\ \rm per\ cent}}$ for LMC X-1 and ${\sim}53{{\ \rm per\ cent}}$ for LMC X-3. We examine the temporal properties of both sources and obtain fractional rms variability of power density spectrum in the frequency range 0.002–10 Hz to be ${\sim}9{}-17{{\ \rm per\ cent}}$ for LMC X-1, and ${\sim}7{}-11{{\ \rm per\ cent}}$ for LMC X-3. The ‘spectro-temporal’ properties indicate both sources are in thermally dominated soft state. By modelling the spectra with relativistic accretion disc model, we determine the mass of LMC X-1 and LMC X-3 in the range $7.64{-}10.00$ and $5.35{-}6.22\, \mathrm{ M}_{\odot }$, respectively. We also constrain the spin of LMC X-1 to be in the range 0.82–0.92 and that of LMC X-3 in 0.22–0.41 with 90 per cent confidence. We discuss the implications of our results in the context of accretion dynamics around the BH binaries and compare it with the previous findings of both sources.

We present the results on broadband 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 modeling 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 (kTbb = 0.37 ± 0.01 keV, Γ ∼ 5.9, $L_{bol}=1\%$ of Eddington luminosity LEdd) and hard state (Γ = 1.64 − 2.22, kTe=4−45 keV, Lbol=1−5% LEdd) that requires a multi-colour disc blackbody model (kTin = 0.54 ± 0.01 keV) occasionally. 1E 1740.7−2942 instead is found only in hard state (Γ=1.67−2.32, kTe=5−16 keV, Lbol=1−2% LEdd). Reflection properties of both sources are studied by applying relativistic reflection model RELXILL to the broadband spectra. Our results from AstroSat and NuSTAR consistently unveiled the presence of a Comptonizing region along with an ionized reflection region (ionization parameter logξ=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 modeling 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.