  Application of multi-resolution analysis adopting wavelets in order to
investigate millisecond aperiodic X-ray variability of Cyg X-1 is presented.
This relatively new approach in a time-series analysis allows us to study both
the significance of any flux oscillations localized in time-frequency space, as
well as the stability of its period. Using the observations of the RXTE/PCA we
analyze archival data from Cyg X-1 during its failed state transition. The
power spectrum of such a state is strongly dominated by a single Lorentzian
peak corresponding to a damped oscillator. Our wavelet analysis presented in
this paper shows the existence of short lasting oscillations without an obvious
trend in the frequency domain. Based on this result, we suggest the
interpretation of the dominant components of Cyg X-1 power spectrum in all
spectral states: (i) a power law component dominating a soft state is due to
accretion rate fluctuations related to MRI instability in the cold disk, (ii)
the low frequency Lorentzian is related to the same fluctuations but in the
inner ion torus, and (iii) the high frequency Lorentzian (dominating the power
spectrum during the failed state transition) is due to the dynamical pulsations
of the inner ion torus.
  The usage of the wavelet analysis as a potential and attractive tool in order
to spot the semi-direct evidences of accretion process onto black-holes is
proposed.
