Revisiting the spin of the stellar-mass black hole MAXI J1820+070 with NICER

The black hole X-ray binary MAXI J1820+070 has been frequently observed by NICER since its outburst in 2018. We report the NICER observations of this black hole (BH) X-ray binary, especially during the soft state, and determine the spin of the BH with the continuum-fitting method. We finally constrain the mean spin of MAXI J1820+070 to be $a_{⁎}$=$0.38\pm 0.12$ only using NICER data in the band of 1 – 10 keV. The measured spin value is basically stable within the narrow range of 0.3 - 0.4 when the accretion disk extends into the innermost stable circular orbit (ISCO). We also check the fitted spin values combined with NICER and Insight-HXMT data from 1 – 25 keV, and find $a_{⁎}$=$0.2\pm 0.1$ which is systematically lower than that based on only NICER observations. We discuss some implications about the evolution of the disk and corona during the soft state through changes of the measured spin and mass accretion rate.