Estimating the Spin of Stellar-Mass Black Holes via 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* ~ 0.75 and ~ 0.65-0.75, respectively, from ASCA and RXTE data. For 4U 1543-47, we estimate a* ~ 0.75-0.85 from RXTE data. Thus, neither black hole has a spin approaching the theoretical maximum a* = 1. 
Rebecca Shafee, Jeffrey E. McClintock, Ramesh Narayan, Shane W. Davis,   Li-Xin Li, Ronald A. Remillard