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

¹ ICFO - Institut de Ciències Fotòniques, avinguda del Canal Olimpic s/n, 08860 Castelldefels (Barcelona), Spain e-mail: gabriele.anzolin@icfo.es
² Dipartimento di Astronomia, Università di Padova, vicolo dell'Osservatorio 3, 35122 Padova, Italy e-mail: [fabrizio.tamburini;antonio.bianchini]@unipd.it
³ INAF - Osservatorio Astronomico di Capodimonte, salita Moiariello 16, 80131 Napoli, Italy e-mail: demartino@oacn.inaf.it

Received: 21 February 2010
Accepted: 31 May 2010

Abstract

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.