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

Based on a spectral analysis of the X-ray continuum, we conclude that the compact primary of the binary X-ray source GRS 1915+105 is a rapidly-rotating Kerr black hole. We find a lower limit on the dimensionless spin parameter of a* greater than 0.98. Our result is robust in the sense that it is independent of the details of the data analysis and insensitive to the uncertainties in the mass and distance of the black hole. Furthermore, the result is based on a fully relativistic accretion-disk model, which includes an advanced treatment of spectral hardening. Our data selection relies on a rigorous and quantitative definition of the thermal state of black hole binaries, which we used to screen all of the available RXTE and ASCA data for the thermal state of GRS 1915+105. We also give an update on published results on the relatively moderate spins that have been reported for three other stellar-mass black holes: a* ~ 0.65-0.80 for GRO J1655-40, a* ~ 0.70-0.85 for 4U 1543-47, and a* less than 0.26 for LMC X-3. Finally, we discuss the significance of our results for relativistic-jet and core-collapse models, and for the detection of gravitational waves