  Recently, wavelets and R/S analysis have been used as statistical tools to
characterize the optical flickering of cataclysmic variables. 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. 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. The X-ray flickering is typically composed of long
time scale events (1.5 < alpha < 3), with very similar strengths in all the
subtypes of cataclysmic variables (-3 < Sigma < -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 Sigma 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. The
similarity of the X-ray flickering in objects of different classes together
with the predominance of a persistent stochastic behavior can be explained it
terms of magnetically-driven accretion processes acting in a considerable
fraction of the analyzed objects.
