Black hole X-ray transients: mass accumulation in the disk - constraints for the viscosity

The outburst cycles of black hole X-ray transients are now generally understood as caused by a thermal instability in the accretion disk, the same mechanism as in dwarf novae outbursts. During quiescence the accretion occurs via a cool disk in the outer region but changes to a coronal flow/ADAF in the inner region. The transition to the coronal flow is caused by evaporation of matter from the cool disk. This process is an important feature for the disk evolution. We point out that if the disk is depleted during the outburst, e.g. by irradiation, its evolution during quiescence is independent of the detailed outburst luminosity decline. The mass accumulation during quiescence has to meet several constraints as the accretion rate, the recurrence time and the total outburst energy. We present a critical discussion of different ways to model X-ray transient outburst cycles and compare with the requirements from observations. For the case of only little mass left over after the outburst the observations indicate a frictional parameter in the cool disk of order alpha(cold) =0.05, similar to that in dwarf nova disks during quiescence, with no need to resort to much lower alpha values of order 0.005.