A physical model for the flickering variability in cataclysmic variables

Abstract

Aperiodic broad-band variability (also known as flickering) is observed throughout all types of accreting compact objects. Many statistical properties of this variability can be naturally explained with the fluctuating accretion disk model, where variations in the mass-transfer rate through the disk are modulated on the local viscous timescale and propagate towards the central compact object. Here, a recently developed implementation of the model is applied for the first time to the time-averaged, high-frequency variability of a cataclysmic variable star (MV Lyrae) observed with the Kepler satellite. A qualitatively good fit to the data is achieved, suggesting the presence of geometrically thick inner flow with large viscosity parameter, extending from $\sim0.12R_{\odot}$ all the way to the white dwarf surface. A simple spectral model of the system suggests that the geometrically thick component would not contribute much to the observed optical flux originating from the geometrically thin outer disk. Instead, X-ray reprocessing from the geometrically thick disk onto the thin disk is proposed as a mechanism to explain the observed variability. Similar flows are also deduced in accreting neutron stars/black holes (X-ray binaries) and Active Galactic Nuclei. Additionally, eclipse mapping studies of cataclysmic variables also seem to suggest the presence of a geometrically extended flow towards the inner-edges of the accretion disk. The fluctuating accretion disk model applied here is encouraging in understanding the origin of flickering in cataclysmic variables, as well as in X-ray binaries and Active Galactic Nuclei, by providing a unifying scheme by which to explain the observed broad-band variability features observed throughout all compact accreting systems.