Estimating the Black Hole Spin for the X-Ray Binary MAXI J1820+070

MAXI J1820+070 is a newly discovered black hole X-ray binary, whose dynamical parameters, namely the black hole mass, the inclination angle, and the source distance, have recently been estimated. Insight-HXMT have observed its entire outburst from 2018 March 14. In this work, we attempted to estimate the spin parameter a (*), using the continuum-fitting method and applying a fully relativistic thin disk model to the soft-state spectra obtained by Insight-HXMT. It is well known that a (*) is strongly dependent on three dynamical parameters in this method, and we have examined two sets of parameters. Adopting our preferred parameters: M = 8.48(-0.72)(+0.79)M(circle dot), i = 63 degrees +/- 3 degrees, and D = 2.96 +/- 0.33 kpc, we found a slowly spinning black hole of a* = 0.14 +/- 0.09 (1 sigma), which gives a prograde spin parameter as the majority of other systems show. It is also possible for the black hole to have a retrograde spin (less than 0) if different dynamical parameters are taken.