Temperature Profiles of Accretion Disks in X-Ray Binaries

Abstract

We present the new results of fitting X-ray spectra of two representative X-ray binaries; X-ray Nova Muscae 1991 and Cyg X-2, by a non-steady disk model, in which a temperature gradient, a ≡ — dlogT/dlogr, is left as a fitting parameter (cf. a = 0.75 in a steady disk model). In the case of Nova Muscae, the derived a value is roughly constant, ~ 0.75, except at the main flare, when a temporarily decreased below 0.75, and at the secondary flare (or the reflare) occuring ~ 70 days after the main peak, when a was transiently increased. Although statistics are poor, these results, if real, would indicate that the disk brightening responsible for the main peak is initiated in the region surrounding the innermost region, while the reflare has its origin in the innermost region of the disk. In the case of Cyg X-2, on the other hand, both a and R _in increase when the system evolves from the horizontal branch to the flaring branch via the normal branch, suggesting the formation of a vertically inflated high-temperature region around a neutron star on the flaring branch