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

Two component X–ray spectra (soft multicolor black body plus harder power law) are frequently observed from accreting black holes. These components are presumably associated with the different parts of the accretion flow (optically thick and optically thin respectively) in the vicinity of the compact source. Timing analysis further suggests that most of the aperiodic variability of the X–ray flux on the short time scales is associated with the harder component. In particular the RXTE spectra of Cygnus X–1 observed during soft state in June 1996 can be well represented as a sum of a constant soft component and strongly variable harder component. We suggest that drastically different amplitudes of variability of these two components are simply related to the very different viscous time scales in the geometrically thin (optically thick) and geometrically thick (optically thin) parts of the accretion flow. In the geometrically thin disks variations of viscosity or mass accretion rate occurring at large radius from the black hole on the local dynamical or thermal time scales do not cause any significant variations of the mass accretion rate at smaller radii due to a very long diffusion time. Any variations on the time scales shorter than the diffusion time scale are effectively dampened. On the contrary such variations can easily survive in the geometrically thick (optically thin) flows and as a result the mass accretion rate in the innermost 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 principle instabilities operate on the short (dynamical or thermal) time scales then the stability of the soft component (originating from the geometrically thin and optically thick flow) and variability of the hard component (coming from the geometrically thick and optically thin flow) are naturally explained. For Cygnus X-1 overall shape of the power density spectra (PDS) in the soft and hard spectral states can be qualitatively explained if the geometrically thin disk is sandwiched by the geometrically thick corona extending in a radial direction up to large distance from the compact object. In the hard state the thin disk is truncated at some distance from the black hole followed by the geometrically thick flow. The break in the PDS is then associated with the characteristic frequencies in the accretion flow at the thin disk truncation radius.