The spin of new black hole candidate: MAXI J1803-298 observed by NuSTAR and NICER

MAXI J1803-298, a newly-discovered Galactic transient, and black hole candidate was first detected by MAXI/GSC on 2021 May 01. 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 NuSTAR and NICER/XTI on the same day over the energy range between 0.7-79.0 keV in SIMS state, and the observed inner radius of the accretion disc is demonstrated to have extended to the ISCO radius (R_in =1.08 R_ISCO). Implementing the relativistic reflection model relxillCp, we found its spin (and the inclination angle i) can be constrained to be close to an extreme value, 0.991 (i ~ 70 °), at 68 per cent confidence interval. The results suggest that MAXI J1803-298 may be a fast-rotating black hole with a large inclination angle.