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. Our spectral model consists of one principal component, a multitemperature blackbody disk model that includes all general relativistic effects, plus standard low-energy absorption and Gaussian line components. For GRO J1655-40, using a luminosity-dependent spectral hardening factor computed for a non-LTE relativistic accretion disk, we estimate a∗ ∼ 0.8 and ∼ 0.7 − 0.75, respectively, from ASCA and RXTE data. For 4U 1543-47, we estimate a∗ ∼ 0.85−0.9 from RXTE data. We conclude that neither black hole is likely to have a spin approaching the theoretical maximum a∗ = 1. On the other hand, the spin of 4U 1543-47 appears to be too large to be explained by disk accretion over the lifetime of the system, which suggests that our measurements are sensitive to the natal spins of these black holes. Subject headings: X-ray: stars — binaries: close — accretion, accretion disks — black hole physics — stars: individual (4U 1543-47, GRO J1655-40 )