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