The spin measurement of MAXI J0637-430: a black hole candidate with high disk density

Abstract

The Galactic black hole candidate MAXI J0637-430 was first discovered by $\textit{MAXI/GSC}$ on 2019 November 02. We study the spectral properties of MAXI J0637-430 by using the archived $\textit{NuSTAR}$ data and $\textit{Swift}$/XRT data. After fitting the eight spectra by using a disk component and a powerlaw component model with absorption, we select the spectra with relatively strong reflection components for detailed X-ray reflection spectroscopy. Using the most state-of-art reflection model $\tt{relxillCp}$, the spectral fitting measures a black hole spin $\textit{a}_{\rm{*}} > 0.72$ and the inclination angle of the accretion disk $i$ = $46.1_{-5.3}^{+4.0}$ degrees, at 90 per cent confidence level. In addition, the fitting results show an extreme supersolar iron abundance. Combined with the fitting results of the reflection model $\tt{reflionx\_hd}$, we consider that this unphysical iron abundance may be caused by a very high density accretion disk ( $n_{\rm{e}} > 2.34 \times 10^{21}$ $\rm{cm}^{-3}$ ) or a strong Fe K$\alpha$ emission line. The soft excess is found in the soft state spectral fitting results, which may be an extra free-free heating effect caused by high density of the accretion disk. Finally, we discuss the robustness of black hole spin obtained by X-ray reflection spectroscopy. The result of relatively high spin is self-consistent with broadened Fe K$\alpha$ line. Iron abundance and disk density have no effect on the spin results.