  The theory of coronal evaporation predicts the formation of an inner hole in
the cool thin accretion disk for mass accretion rates below a certain value (~
1/50 of the Eddington mass accretion rate) and the sudden disappearance of this
hole when the mass accretion rate rises above that value. The inner edge of the
standard thin disk then suddenly shifts inward from about a few hundred
Schwarzschild radii to the last stable orbit. This appears to quantitatively
account for the observed transitions between hard and soft spectral states at
critical luminosities. Due to the evaporation process the matter accreting in
the geometrically thin disk changes to a hot coronal flow which proceeds
towards the black hole as an advection-dominated accretion flow (ADAF; for a
review see Narayan et al. 1998).
