Estimating the Spins of Stellar-Mass Black Holes by Fitting Their Continuum Spectra

We have used the Novikov-Thorne thin disk model to fit the continuum X-ray spectra of three transient black hole X-ray binaries in the thermal state. From the fits we estimate the dimensionless spin parameters of the black holes to be: 4U 1543-47, a* = a/M = 0.7-0.85; GRO J1655-40, a* = 0.65-0.8; GRS 1915+105, a* = 0.98-1. We plan to expand the sample of spin estimates to about a dozen over the next several years. Some unresolved theoretical issues are briefly discussed.