XMM-Newton observation of V1504 Cyg as a probe for the existence of an evaporated corona

Abstract

Aims: We present an analysis of an XMM-Newton observation of the dwarf nova V1504 Cyg during the decline from an outburst. Our goal is to search for evidence of an evaporated X-ray corona. Such a corona can be understood as an optically thin and geometrically thick disc around the central part of an optically thick and geometrically thin disc.
Methods: We study the X-ray spectra of a dwarf nova using a cooling-flow model and the evolution in the amplitude of variability and power density spectra in the UV and X-ray.
Results: The X-ray (pn) count rate increases from initially around 0.03 cps to 0.17 cps, with a harder spectrum and a higher degree of variability. Meanwhile, the OM/UVW1 light curve follows a slow decline with a decreasing amplitude of variability. Next, we split the X-ray data into two segments and analysed them separately. Both were described by a cooling-flow model, while the first low-luminosity segment required an additional power-law component, suggesting the presence of a wind. A spectral fitting revealed a higher temperature for the second brighter segment. A timing analysis revealed a potential break frequency at log(f/Hz) = −3.02 during the decline towards the quiescence. This detection is in agreement with optical data from Kepler observations.
Conclusions: The X-ray nature of the break frequency supports the innermost parts of the disc as source of the variability. Moreover, a similar frequency was observed in several other cataclysmic variables. Thus, a sandwich model where a geometrically thick corona surrounds the geometrically thin disc is a plausible accretion configuration.