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

TL;DR

This study analyzes broadband X-ray spectra of the black hole binary MAXI J1820+070 using AstroSat and NuSTAR, revealing details about the accretion disk, corona properties, and black hole mass during the hard state.

Contribution

It provides a comprehensive spectral analysis combining data from AstroSat and NuSTAR, introducing a model with multiple Comptonisation components and estimating the black hole mass.

Findings

Low temperature accretion disk ($kT_{in} \\sim 0.3$ keV)

Inhomogeneous corona with decreasing temperature at higher altitudes

Black hole mass estimated between 6.7 and 13.9 solar masses

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 ($kT_{\rm in} \sim 0.3$ keV), iron overabundance ($A_{\rm Fe} \sim 4-5$ solar), a short lamp-post corona height ($h \lesssim 8 R_{\rm g}$), and a high corona temperature ($kT_{\rm e} \sim 115-150$ keV). Addition of a second Comptonisation component leads to a significantly better fit, with the $kT_{\rm e}$ 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 $6.7-13.9 \ M_{\odot}$, consistent with independent measurements reported in the literature.