The disc instability model for X-ray transients: Evidence for truncation and irradiation

We study the prospect of explaining the outbursts of Soft X-ray Transients (SXTs) by the thermal-viscous instability in a thin disc. This instability is linked to hydrogen ionization and is significantly changed when irradiat ion of the disc by X-rays from the inner regions is included. We present th e first numerically reliable, physically consistent calculations of the out burst cycles which include the effects of accretion disc irradiation. The d ecay from outburst is governed by irradiation, as pointed out by King & Rit ter (1998), leading to slow exponential decays. At the end of the outburst, the disc is severely depleted, which lengthens the time needed to rebuild mass to the critical density for an outburst. Despite this, the long recurr ence times and quiescent X-ray luminosities of SXTs still require the inner disc to be replaced by another type of ow in quiescence, presumably a hot advection dominated accretion ow (ADAF). We include the effects of such tru ncation and show that the resulting lightcurves are conclusively similar to those of SXTs like A0620-00. We conclude that the two-alpha disc instabili ty model provides an adequate description of the outbursts of SXTs when bot h truncation and irradiation are included. The values for the viscosities i n outburst and in quiescence are comparable to those used in CVs. We discus s the model in the context of present observations.