A Model for the X-Ray Transition in Galactic Black Hole Candidate Objects

Abstract

Stellar mass black hole candidates (BHCs) have long been known to exhibit the soft and hard spectral states. Occasional state transitions are observed in some BHCs, although their mechanisms are not yet known. We perform the time evolutionary calculations of the standard-type accretion disks in order to see how they evolve when mass flow rates are close to the critical rate, ~LEdd/c2 (with LEdd being the Eddington luminosity and c being the speed of light). Interestingly, we find that for moderately large viscosity parameters (α 0.3), the inner parts make a transition from the standard branch to the optically thin, advection-dominated branch. This is true for the following reason: the standard branch is not connected to the optically thick, advection-dominated branch at small radii for α 0.3, so the disk should soon become optically thin. The consequence is a composite disk configuration, in which the inner, optically thin, hot part is separated by the outer, optically thick, cool part. In contrast, when α is smaller (α 0.1), we basically confirmed the previous claim by F. Honma and others; the disks undergo limit-cycle oscillations between the standard branch and the optically thick, advection-dominated branch, thereby producing burstlike X-ray variations. Such limit-cycle oscillations are not generally observed in the usual BHCs (except for the microquasar GRS 1915+105), implying that α is relatively large in the usual hot disks.