  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.
Most of the aperiodic variability of the X-ray flux on the short time scales is
associated with the 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 and geometrically
thick 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 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 primary instabilities operate on the short 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.
