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 novae V1504 Cyg during the decline from an outburst. Our goal is to search for evidence for an evaporated X-ray corona. Such a corona can be understood as an optically thin geometrically thick disc around a central part of an optically thick geometrically thin disc. METHODS: We study the X-ray spectra using a cooling flow model and the evolution of the amplitude of variability and power density spectra in UV and X-rays. 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 decreasing amplitude of variability. For further study we split the X-ray data into two parts, and analysed them separately. Both parts are described by a cooling flow model, while the first low luminosity part requires an additional power law component suggesting presence of a wind. Spectral fitting revealed a higher temperature during the second brighter part. Timing analysis reveals a potential break frequency at log(f/Hz) = -3.02 during decline towards the quiescence. This detection agrees 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 and a sandwich model where a geometrically thick corona surrounds the geometrically thin disc is a possible accretion configuration.