On the Temperature Profile of Radiatively Efficient Geometrically Thin Disks in Black Hole Binaries with the ASCA GIS

摘要

X-ray spectra of black hole binaries in the standard high/soft state were studied comprehensively using ASCA GIS data and, partially, RXTE PCA data. A mathematical disk model was applied to several black hole binaries to see whether the observed accretion disk temperature profile was consistent with that expected from the standard accretion disk model. This model is called a p-free disk and assumes that the spectrum is composed of multitemperature blackbody emission, where the local temperature T(r) at radius r is given by T(r) ∝ r-p, with p being a positive free parameter. The standard disk, where the gravitational energy of the accreted matter is effectively released as blackbody radiation, roughly requires p ≈ 3/4, while a small deviation is expected depending on the inner boundary conditions, general relativistic effects, and disk vertical structures. Our sample objects included LMC X-1, LMC X-3, XTE J2012+381, and GRO J1655-40. During the ASCA observations, these black hole binaries showed characteristics of the standard high/soft state. Under the standard modeling of high-state black hole binaries, the sources show disk inner temperatures of 0.76-1.17 keV, a disk fraction of the total 0.7-10 keV flux of 54%-98%, and absorption columns of (0.7-12) × 1021 cm-2. The best-fit values of p were found in the range 0.6-0.8, and the standard value of p = 3/4 was accepted for all the sources. The obtained values of p are also compared with those expected for the standard accretion disk in the Schwarzschild metric using the so-called GRAD model. The observed p-values were indeed found to be consistent with those expected from the standard accretion disk in the Schwarzschild metric.