Multitemperature Blackbody Spectrum of a Thin Accretion Disk around a Kerr Black Hole: Model Computations and Comparison with Observations
Abstract
We use a raytracing technique to compute the observed spectrum of a thin accretion disk around a Kerr black hole. We include all relativistic effects such as framedragging, Doppler boost, gravitational redshift, and bending of light by the gravity of the black hole. We also include selfirradiation of the disk as a result of light deflection. Assuming that the disk emission is locally blackbody, we show how the observed spectrum depends on the spin of the black hole, the inclination of the disk, and the torque at the inner edge of the disk. We find that the effect of a nonzero torque on the spectrum can, to a good approximation, be absorbed into a zerotorque model by adjusting the mass accretion rate and the normalization. We describe a computer model, called KERRBB, which we have developed for fitting the spectra of black hole Xray binaries. Using KERRBB within the Xray data reduction package XSPEC, and assuming a spectral hardening factor f_{col}=1.7, we analyze the spectra of three black hole Xray binaries: 4U 154347, XTE J1550564, and GRO J165540. We estimate the spin parameters of the black holes in 4U 154347 and GRO J165540 to be a/M~0.6 and ~0.60.7, respectively. If f_{col}~1.51.6, as in a recent study, then we find a/M~0.70.8 and ~0.80.9, respectively. These estimates are subject to additional uncertainties in the assumed black hole masses, distances, and disk inclinations.
 Publication:

The Astrophysical Journal Supplement Series
 Pub Date:
 April 2005
 DOI:
 10.1086/428089
 arXiv:
 arXiv:astroph/0411583
 Bibcode:
 2005ApJS..157..335L
 Keywords:

 Accretion;
 Accretion Disks;
 Black Hole Physics;
 Radiation Mechanisms: Thermal;
 XRays: Binaries;
 Astrophysics
 EPrint:
 75 pages, including 14 figures