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

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.