| Abstract: In this work, spectro-temporal studies on three BHXBs have been carried out primarily using AstroSat's SXT and LAXPC. Of the three sources, one is a transient source, MAXIJ1820+070, and the others are persistent sources, LMCX-1 and 4U1957+115. For the MAXI source, combined spectra (0.7–80.0 keV) were well modeled using disk blackbody emission, thermal Comptonization, and a reflection component. Source was found to be in its hard spectral state (Γ = 1.61) with a cool disk (kTin = 0.22 keV). We modelled observed flux variability using a single-zone stochastic propagation model at different frequencies using LAXPC data. Additionally, we confirm the detection of a QPO at 47.7 mHz for the first time using AstroSat.
Next, we modelled the combined SXT and LAXPC spectra (0.7-30.0 keV) of LMC X-1 with diskbb, simpl and Tbabs models. Spectral analysis revealed that the source was in its high-soft state (Γ = 2.67) with a hot disc (kTin = 0.86). Thermal disk emission was fitted with a relativistic model (kerrbb) and the spin of the black hole was estimated to be 0.93 (statistical errors) through X-ray continuum-fitting, which agrees with the previous results.
We then performed multi-mission spectral study of 4U 1957+115, using AstroSat, Swift, and NuSTAR. Modelling with a disk emission, thermal Comptonization, and blurred reflection components revealed that the source was in the high-soft state (Γ = 2.6) with the disc flux ∼87% of the total. We found evidence that either the inner disc radius varied by ∼25% or the colour hardening factor changed by ∼12%. Fixing the distance to 10 kpc and using a relativistic accretion disk model, constrained the black hole mass to 6 M⊙ and inclination angle to 72°. A positive correlation is detected between the accretion rate and inner radii or equivalently between the accretion rate and colour factor. |
