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.

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. Unlike many other BHXBs, the black hole mass, source distance and binary orbital inclination are well established. In this work, we report the results of simultaneous broadband spectral studies of LMC X-1 carried out using the data from Soft X-ray Telescope and Large Area X-ray Proportional Counter aboard AstroSat as observed on 2016 November 26th and 2017 August 28th. The combined spectrum was modelled with a multicolour blackbody emission (diskbb), a Gaussian along with a Comptonization component (simpl) in the energy range 0.7−30.0 keV. The spectral analysis revealed that the source was in its high soft state (Γ = 2.67$^{+0.24}_{-0.24}$ and Γ = 2.12$^{+0.19}_{-0.20}$) with a hot disc (kTin = 0.86$^{+0.01}_{-0.01}$ and kTin = 0.87$^{+0.02}_{-0.02}$). Thermal disc emission was fit with a relativistic model (kerrbb) and spin of the black hole was estimated to be 0.93$^{+0.01}_{-0.01}$ and 0.93$^{+0.04}_{-0.03}$ (statistical errors) for the two Epochs through X-ray continuum-fitting, which agrees with the previous results.

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 blackbody emission, thermal Comptonization and a reflection component. The spectral analysis revealed that the source was in its hard spectral state (Γ = 1.61) with a cool disk (kT in = 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.

We report spectral analysis of the persistent black hole X-ray binary, 4U 1957+115, using AstroSat, Swift and NuSTAR observations carried out between 2016-2019. Modelling with a disk emission, thermal Comptonization and blurred reflection components revealed that the source was in the high soft state with the disk flux ∼87 per cent of the total and high energy photon index ∼2.6. There is an evidence that either the inner disk radius varied by ∼25 per cent or the colour hardening factor changed by ∼12 per cent. The values of the inner disk radius imply that for a non-spinning black hole, the black hole mass is <7 M⊙ and the source is located >30 kpc away. On the other hand, a rapidly spinning black hole would be consistent with the more plausible black hole mass of <10 M⊙ and a source distance of ∼10 kpc. 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.

4U 1543-47 underwent its brightest outburst in 2021 after two decades of inactivity. During its decay phase, AstroSat conducted nine observations of the source spanning from July 1st to September 26th, 2021. The first three observations were performed with an offset of 40′ with AstroSat/LAXPC, while the remaining six were on-axis observations. In this report, we present a comprehensive spectral analysis of the source as it was in the High/Soft state during the entire observation period. The source exhibited a disk-dominated spectra with a weak high-energy tail (power-law index ≥2.5) and a high inner disk temperature (∼0.84 keV). Modelling the disk continuum with non-relativistic and relativistic models, we find inner radius to be significantly truncated at >10 Rg. Alternatively, to model the spectral evolution with the assumption that the inner disk is at the ISCO, it is necessary to introduce variation in the spectral hardening in the range ∼1.5-1.9.