Mechanisms for the Outbursts of Soft X-ray Transients

We show that the Keplerian thin disk in quiescent Soft X-ray Transients cannot extend down to the last stable orbit around the central black hole. We analyse the properties of the Narayan, McClintock & Yi (1996) model of quiescent Soft X-ray Transients in which the cold Keplerian disk has its inner edge at a large transition radius and transforms to a hot, advection-dominated flow on the inside. We show that outbursts of transient sources could be triggered in this model either by a pure thermal accretion disk instability or by a disk instability generated by an enhanced mass transfer from the stellar companion. Both mechanisms operate in the outer thin disk and could be at work, either in different systems or in the same system at different epochs, depending on the mass transfer rate and the value of the viscosity parameter $\alpha_t$ of the thin disk. We show that the recurrence time between outbursts in SXTs can be explained with values of $\alpha_t$ similar to these required by the dwarf nova disk instability model instead of the unreasonably low values needed in the model in which the thin disks extends down to the last stable orbit. We extend the Narayan, McClintock & Yi (1996) model to the case when the outer disk is non-stationary. We show that such disk is too cold to account for the observed UV flux. This difficulty is common to all models in which the outer disk is assumed to be optically thick.