A spectral study of the black hole X-ray binary MAXI J1820+070 with AstroSat and NuSTAR

Abstract

MAXI J1820+070 is a newly discovered transient black hole X-ray binary, which showed several spectral and temporal features. In this work, we analyse the broadband X-ray spectra from all three simultaneously observing X-ray instruments onboard AstroSat, as well as contemporaneous X-ray spectra from NuSTAR, observed during the hard state of MAXI J1820+070 in March 2018. Implementing a combination of multi-colour disc model, relativistic blurred reflection model relxilllpCp and a distant reflection in the form of xillverCp, we achieve reasonable and consistent fits for AstroSat and NuSTAR spectra. The best-fit model suggests a low temperature disc ($k{T}_{\mathrm{in}}\sim 0.3$ keV), iron overabundance ($A_{\mathrm{Fe}}\sim 4-5$ solar), a short lamp-post corona height ($h\lesssim 8R_{\mathrm{g}}$), and a high corona temperature ($k{T}_{\mathrm{e}}\sim 115-150$ keV). Addition of a second Comptonisation component leads to a significantly better fit, with the $k{T}_{\mathrm{e}}$ of the second Comptonisation component being $\sim 14-18$ keV. Our results from independent observations with two different satellites in a similar source state indicate an inhomogeneous corona, with decreasing temperature attributed to increasing height. Besides, utilising the broader energy coverage of AstroSat, we estimate the black hole mass to be $6.7-13.9M_{\odot }$, consistent with independent measurements reported in the literature.