Broadband reflection spectroscopy of MAXI J1535-571 using AstroSat: Estimation of black hole mass and spin

We report the results from $\mathit{\text{AstroSat}}$ observations of the transient Galactic black hole X-ray binary MAXI J1535-571 during its hard-intermediate state of the 2017 outburst. We systematically study the individual and joint spectra from two simultaneously observing $\mathit{\text{AstroSat}}$ X-ray instruments, and probe and measure a number of parameter values of accretion disc, corona and reflection from the disc in the system using models with generally increasing complexities. Using our broadband ($1.3-70$ keV) X-ray spectrum, we clearly show that a soft X-ray instrument, which works below $\sim 10-12$ keV, alone cannot correctly characterize the Comptonizing component from the corona, thus highlighting the importance of broadband spectral analysis. By fitting the reflection spectrum with the latest version of the $\text{textsc}relxill$ family of relativistic reflection models, we constrain the black hole’s dimensionless spin parameter to be $0.67_{-0.04}^{+0.16}$. We also jointly use the reflection spectral component ($\text{textsc}relxill$) and a general relativistic thin disc component ($\mathtt{\text{Kerrbb}}$), and estimate the black hole’s mass and distance to be $10.39_{-0.62}^{+0.61}{M}_{\odot }$ and $5.4_{-1.1}^{+1.8}$ kpc respectively.