Wavelet analysis of millisecond variability of Cygnus X-1 during its failed state transition

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.