Abstract
The author has constructed a model for Cygnus X-1 consisting of an outer optically thick and geometrically thin disc, an inner optically thin and geometrically thick disc and above it, a coronal region that scatters photons from the outer disc to the inner disc, where they are inverse comptonized. This model reproduces the observed spectrum of Cygnus X-1 quite well. Using a local stability criterion, and taking into account the contribution of the turbulent pressure pulsations in hydrostatic equilibrium, it is shown that the inner region is thermally unstable against small perturbations in the radial direction. For the outer disc it is seen that instability sets in the disc as long as the radiation pressure exceeds a critical value. This value, however, depends on the turbulent Mach number.
