We study the transition from the geometrically thin disk to the hot coronalflow for accretion onto black holes. The efficiency of evaporation determinesthe truncation of the geometrically thin disk as a function of the black holemass and the mass flow rate in the outer disk. The physics of the evaporationwas already described in detail in earlier work (Meyer et al. 2000b). We shownow that the value of the viscosity parameter for the coronal gas has a stronginfluence on the evaporation efficiency. For smaller values of the viscosityevaporation is less efficient. For a given mass flow rate from outside thegeometrically thin disk then extends farther inward. Spectral transitionsbetween soft and hard states are then expected for different mass flow rates inthe outer disk. The physics is the same for the cases of stellar andsupermassive black holes systems.
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