Estimating the Spins of Stellar-Mass Black Holes

We describe a program that we have embarked on to estimate the spins of stellar-mass black holes in X-ray binaries. We fit the continuum X-ray spectrum of the radiation from the accretion disk using the standard thin disk model, and extract the dimensionless spin parameter a* = a/M of the black hole as a parameter of the fit. We have obtained results on three systems, 4U 1543-47 (a* = 0.7-0.85), GRO J1655-40 (0.65-0.8), and GRS 1915+105 (0.98-1), and have nearly completed analysis of two additional systems. We anticipate expanding the sample of spin estimates to about a dozen over the next several years.