The spin of the black hole GRS 1716-249 determined from the hard intermediate state

Abstract: We present three simultaneous/quasi-simultaneous NuSTAR and Swift datasets of the black hole GRS 1716-249 in its hard intermediate state. The accretion disk in this state may have reached the innermost stable circular orbit, and the NuSTAR spectra show a broad relativistic iron line and a strong Compton hump. To measure the black hole spin, we construct a joint model consisting of a relativistic disk model kerrbb and a reflection model relxill, to fit the continuum and the reflection components, respectively. By applying this model to each dataset independently, a consistent result is obtained on the black hole spin and the disk inclination. The black hole spin is a* >~ 0.92, and the inclination angle (i) is around 40-50 degree, based on the measurements of all datasets. In the third dataset, a high black hole mass (M_BH) is strongly disfavored by the spectral fits. By unfreezing the black hole mass, we find a*>0.92, i=49.9^{+1.0}_{-1.3} degree and M_BH<8.0 Msun, at a 90% confidence level. Considering the lower limit derived from a previous optical constraint, M_BH is in a range of 4.9-8.0 Msun.