Soft state of Cygnus X-1: stable disc and unstable corona

Two-component X-ray spectra (soft multicolour black-body and harder power l aw) are frequently observed from accreting black holes. These components ar e presumably associated with the different parts of the accretion flow (opt ically thick and optically thin respectively) in the vicinity of the compac t source. Most of the aperiodic variability of the X-ray flux on the short time-scales is associated with the harder component. We suggest that drasti cally different amplitudes of variability of these two components are simpl y related to the very different viscous time-scales in the geometrically th in and geometrically thick parts of the accretion flow. In the geometrically thin discs, variations of viscosity or mass accretion rate occurring at large radius from the black hole on the local dynamical o r thermal time-scales do not cause any significant variations of the mass a ccretion rate at smaller radii because of a very long diffusion time. Any v ariations on the time-scales shorter than the diffusion time-scale are effe ctively dampened. On the contrary such variations can easily survive in the geometrically thick flows and as a result the mass accretion rate in the i nnermost region of the flow will reflect modulations of the mass accretion rate added to the flow at any distance from the black hole. Therefore if pr imary instabilities operate on the short time-scales then the stability of the soft component (originating from the geometrically thin and optically t hick flow) and variability of the hard component (coming from the geometric ally thick and optically thin flow) are naturally explained. For Cygnus X-1, the overall shape of the power density spectra (PDS) in the soft and hard spectral states can be qualitatively explained if the geomet rically thin disc is sandwiched by the geometrically thick corona extending in a radial direction up to a large distance from the compact object. In t he hard state the thin disc is truncated at some distance from the black ho le followed by the geometrically thick flow. The break in the PDS is then a ssociated with the characteristic frequencies in the accretion flow at the thin disc truncation radius.