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, \u3b1 and \u3a3, 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 \u2a89 \u3b1 \u2a89 3), with very similar strengths in all the subtypes of cataclysmic variables (-3 \u2a89 \u3a3 \u2a89 -1.5). The X-ray data are distributed in a much smaller area of the \u3b1 - \u3a3 parameter space with respect to those obtained with optical light curves. The tendency of the optical flickering in magnetic systems to show higher \u3a3 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