Broad-band reflection spectroscopy of MAXI J1535-571 using AstroSat: estimation of black hole mass and spin

We report the results from 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 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 broad-band (1.3-70 keV) X-ray spectrum, we clearly show that a soft X-ray instrument, which works below 10-12 keV, alone cannot correctly characterize the Comptonizing component from the corona, thus highlighting the importance of broad-band spectral analysis. By fitting the reflection spectrum with the latest version of the RELXILL family of relativistic reflection models, we constrain the black hole's dimensionless spin parameter to be 0.67^{+0.16}_{-0.04}. We also jointly use the reflection spectral component (RELXILL) and a general relativistic thin disc component (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.