TIME-DEPENDENT DISK ACCRETION IN X-RAY NOVA MUSCAE 1991

We propose a new model for X-ray spectral fitting of binary black hole candidates. In this model, it is assumed that X-ray spectra are compo sed of a Comptonized blackbody (hard component) and a disk blackbody s pectra (soft component), in which the temperature gradient of the disk , q = -d log T/d log r, is left as a fitting parameter. With this mode l, we have fitted X-ray spectra of X-ray Nova Muscae 1991 obtained by Ginga. The fitting shows that a hot cloud, which Compton up-scatters s oft photons from the disk, gradually shrank and became transparent aft er the main peak. The temperature gradient turns out to be fairly cons tant and is q approximately 0.75, the value expected for a Newtonian d isk model. To reproduce this value with a relativistic disk model, a s mall inclination angle, i congruent-to 0-degrees-15-degrees, is requir ed. It seems, however, that the q-value temporarily decreased below 0. 75 at the main flare, and q increased in a transient fashion at the se cond peak (or the reflare) occurring approximately 70 days after the m ain peak. Although statistics are poor, these results, if real, would indicate that the disk brightening responsible for the main and second ary peaks are initiated in the relatively inner portions of the disk.