The Spin of the Near‐Extreme Kerr Black Hole GRS 1915+105

Based on a spectral analysis of the X-ray continuum that employs a fullyrelativistic accretion-disk model, we conclude that the compact primary of thebinary X-ray source GRS 1915+105 is a rapidly-rotating Kerr black hole. We finda lower limit on the dimensionless spin parameter of a* greater than 0.98. Ourresult is robust in the sense that it is independent of the details of the dataanalysis and insensitive to the uncertainties in the mass and distance of theblack hole. Furthermore, our accretion-disk model includes an advancedtreatment of spectral hardening. Our data selection relies on a rigorous andquantitative definition of the thermal state of black hole binaries, which weused to screen all of the available RXTE and ASCA data for the thermal state ofGRS 1915+105. In addition, we focus on those data for which the accretion diskluminosity is less than 30% of the Eddington luminosity. We argue that theselow-luminosity data are most appropriate for the thin alpha-disk model that weemploy. We assume that there is zero torque at the inner edge of the disk, asis likely when the disk is thin, although we show that the presence of asignificant torque does not affect our results. Our model and the model of therelativistic jets observed for this source constrain the distance and blackhole mass and could thus be tested by determining a VLBA parallax distance andimproving the measurement of the mass function. Finally, we comment on thesignificance of our results for relativistic-jet and core-collapse models, andfor the detection of gravitational waves.