Estimating the spin of stellar-mass black holes by spectral fitting of the X-ray continuum

We fit X-ray spectral data in the thermal-dominant, or high-soft, state of two dynamically confirmed black holes, GRO J1655-40 and 4U 1543-47, and estimate the dimensionless spin parameters a* a/M of the two holes. For GRO J1655-40, using a spectral hardening factor computed for a non-LTE relativistic accretion disk, we estimate a*similar to 0.75 and a*similar to 0.65-0.75, respectively, from ASCA and RXTE data. For 4U 1543-47, we estimate a*similar to 0.75-0.85 from RXTE data. Thus, neither black hole has a spin approaching the theoretical maximum a* = 1.