  A phenomenological model of X-ray variability of accreting black holes is
considered, where the variable emission is attributed to multiple active
regions/perturbations moving radially towards the central black hole. The hard
X-rays are produced by inverse Compton upscattering of soft photons coming from
reprocessing/thermalization of the same hard X-rays. The heating rate of the
Comptonizing plasma is assumed to scale with the rate of dissipation of
gravitational energy while the supply of soft photons is assumed to diminish
towards the center. Two scenarios are considered: (1) an inner hot flow with
outer truncated standard accretion disc and (2) an accretion disc with an
active corona and a thick hot ionized skin. A variant of the model is also
considered, which is compatible with the currently discussed multi-Lorentzian
description of power spectral densities of X-ray lightcurves.
  In the inner hot flow scenario the model can reproduce the observed Fourier
frequency resolved spectra observed in X-ray binaries, in particular the
properties of the reprocessed component as functions of Fourier frequency. In
the accretion disc with ionized skin scenario the reduction of soft photons due
to the ionized skin is insufficient to produce the observed characteristics.
