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

We present an application of multiresolution analysis adopting wavelets in order to investigate the millisecond aperiodic X-ray variability of Cygnus X-1. 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 and the stability of its period. Using the observations of the Rossi X-ray Timing Explorer Proportional Counter Array, we analyse 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 the 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 magnetorotational instability in the cold disc; (ii) the low-frequency Lorentzian is related to the same fluctuations but in the inner ion torus; (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. We propose the use of wavelet analysis as a potential and attractive tool in order to spot semi-direct evidence of the accretion process on to black holes.