On the Accretion Instability in Soft X-Ray Transients

Dwarf nova outbursts are likely caused by a thermal-viscous instability in the accretion disk around the white dwarf, which can occur if the mass transfer rate, , is below a critical value (which depends on orbital period). Based on data for soft X-ray transients and persistent low-mass X-ray binaries with known distances and orbital periods, Porb, I show that for these systems the distributions in a (Porb, ) diagram can be understood with the dwarf nova instability criterion, if that is adapted to include X-ray heating of the disk. This supports the idea that the disk instability model applies to soft X-ray transients, and shows that X-ray heating must be included in modeling the outbursts.