The Spin of New Black Hole Candidate: MAXI J1803-298 Observed by NuSTAR and NICER

Abstract: MAXI J1803-298, a newly-discovered Galactic transient and black hole candidate, was first detected by \emph{MAXI}/GSC on May 1st, 2021. In this paper, we present a detailed spectral analysis of MAXI J1803-298. Utilizing the X-ray reflection fitting method, we perform a joint fit to the spectra of MAXI J1803-298, respectively, observed by \emph{NuSTAR} and \emph{NICER}/XTI on the same day over the energy range between 0.7-79.0 keV, and found its spin (and the inclination angle i) can be constrained to be close to an extreme value, 0.991 ($i\sim$ $70 ^{\circ}$), at 68\% confidence interval. The results suggest that MAXI J1803-298 may be a fast-rotating black hole with a large inclination angle.