Abstract
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 a* ~ 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.
Highlights
An astrophysical black hole is completely defined by two numbers that specify its mass and spin
There is agreement with the simultaneous RXTE observation done on 1997 Feb 26
Using the observed luminosities of J1655 during the ASCA 1995 and 1997 observations, viz., log l ∼ −0.85, − 1.02, we have estimated the respective values of fcol according to the ST95 model
Summary
An astrophysical black hole is completely defined by two numbers that specify its mass and spin. We estimate a∗ of two black hole binaries by fitting their X-ray thermal continuum spectra using a fully relativistic model of a thin accretion disk around a Kerr. The model includes all relativistic effects such as frame-dragging, Doppler boosting, gravitational red shift, and bending of light by the gravity of the black hole. It includes self-irradiation of the disk, the effects of limb darkening, and a spectral hardening factor fcol to relate the color temperature Tcol and the effective temperature Teff of the disk emission: fcol = Tcol/Teff (Shimura & Takahara 1995, hereafter ST95; Merloni, Fabian & Ross 2000; Davis et al 2005, hereafter D05).
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