Disk instability-induced hard-soft transition in soft X-ray transients

Soft X-ray transients that are black-hole candidates (BHCs) exhibit hysteretic spectral transitions during outbursts; hard X-ray peaks generally precede soft X-ray peaks. To account for such behavior we propose a new model, in which the disk behavior is distinct across a critical radius. The outer portions suffer a thermal limit-cycle instability owing to partial ionization of hydrogen and helium, thereby modulating mass flow into the inner portions. In order to produce weak X-rays during the quiescence, however, the instability should be suppressed in the inner portions, probably due to small optical depths. Instead, the inner portions undergo forced bimodal transitions between the optically thick and thin branches in response to variable mass input from the outer portions. The inner part is on the optically thin, advection-dominated branch in the quiescence and rise phases, thus producing hard power-law radiation, while it is on the optically thick branch in the early decay phase, predominantly emitting soft radiation. Our model can basically account for the observations of black-hole transients.