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 .