Spectral properties of soft X-ray transient MAXI J0637$-$430 using AstroSat

Abstract

Soft X-ray transients are systems that are detected when they go into an outburst, wherein their X-ray luminosity increases several orders of magnitude. These outbursts are markers of the poorly understood change in the spectral state of these systems from low/hard state to high/soft state. We report the spectral properties of one such soft X-ray transient: MAXI J0637$-$430, with data from the \textit{SXT} and \textit{LAXPC} instruments on-board \textit{AstroSat} mission. The source was observed for a total of $\sim$ 60 ks over two observations on 8$^{th}$ and 21$^{st}$ November, 2019 soon after its discovery. Flux resolved spectral analysis of the source indicates the presence of a multi-colour blackbody component arising from the accretion disk and a thermal Comptonization component. The stable low temperature ($\sim$ 0.55 $keV$) of the blackbody component, points to a cool accretion disk with an inner disk radius of the order of a few hundred $km$. In addition, we report the presence of a relativistically broadened Gaussian line at 6.4 $keV$. The disk dominated flux and photon power law index of $\gtrapprox 2$ and a constant inner disk radius indicate the source to be in the soft state. From the study we conclude that MAXI J0637$-$430 is a strong black hole X-ray binary candidate.