Confirming the spin parameter of the black hole in Cygnus X-1 using the Insight-HXMT

Abstract

Recently, the system parameters of Cygnus X-1, i.e., the black hole mass M, the orbital inclination i and the source distance D, have been updated. As the input constraints of the continuum-fitting method, it is necessary to refine the spin with these new parameters. The Hard X-ray Modulation Telescope (HXMT, also named Insight) was successfully launched on June 15th, 2017 in China. In this work, we analyzed spectra obtained by the Insight-HXMT during 2017-2018 and re-estimated the spin via the continuum-fitting method. We first re-estimated the spin using old parameters and found the dimensionless spin parameter to be a⁎>0.934 (3σ), which is consistent with previous measurements both from the continuum-fitting and iron-line fitting methods (Gou et al. 2011, Gou et al. 2014; Fabian et al. 2012, Walton et al. 2016, Tomsick et al. 2018). Then, when the new parameters were applied, we obtained an increased spin: a⁎>0.967 (3σ), which is also consistent with the recent measurement (Zhao et al. 2020, submitted to ApJ) and also confirms an extreme Kerr black hole lying in this system.