Wavelet and R/S analysis of the X-ray flickering of cataclysmic variables

Context. Recently, wavelets and R/S analysis have been used as statistical tools to characterize the optical flickering of cataclysmic variables. Aims: Here we present the first comprehensive study of the statistical properties of X-ray flickering of cataclysmic variables in order to link them with physical parameters. Methods: We analyzed a sample of 97 X-ray light curves of 75 objects of all classes observed with the XMM-Newton space telescope. By using the wavelets analysis, each light curve has been characterized by two parameters, α and Σ, that describe the energy distribution of flickering on different timescales and the strength at a given timescale, respectively. We also used the R/S analysis to determine the Hurst exponent of each light curve and define their degree of stochastic memory in time. Results: The X-ray flickering is typically composed of long time scale events (1.5 ⪉ α ⪉ 3), with very similar strengths in all the subtypes of cataclysmic variables (-3 ⪉ Σ ⪉ -1.5). The X-ray data are distributed in a much smaller area of the α - Σ parameter space with respect to those obtained with optical light curves. The tendency of the optical flickering in magnetic systems to show higher Σ values than the non-magnetic systems is not encountered in the X-rays. The Hurst exponents estimated for all light curves of the sample are larger than those found in the visible, with a peak at 0.82. In particular, we do not obtain values lower than 0.5. The X-ray flickering presents a persistent memory in time, which seems to be stronger in objects containing magnetic white dwarf primaries. Conclusions: The similarity of the X-ray flickering in objects of different classes together with the predominance of a persistent stochastic behavior can be explained in terms of magnetically-driven accretion processes acting in a considerable fraction of the analyzed objects.