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

¹ Advanced Technologies Research Institute, Faculty of Materials Science and Technology in Trnava, Slovak University of Technology in Bratislava, Bottova 25, 917 24 Trnava, Slovakia
e-mail: andrej.dobrotka@stuba.sk
² XMM-Newton Science Operations Centre, European Space Astronomy Centre, Camino Bajo del Castillo s/n, Urb. Villafranca del Castillo, 28692 Villanueva de la Cañada, Madrid, Spain
³ Department of Mathematics and Physics E. De Giorgi, University of Salento, Via per Arnesano, CP 193, 73100 Lecce, Italy
⁴ INFN, Sez. di Lecce, Via per Arnesano, CP 193, 73100 Lecce, Italy

Received: 23 November 2021
Accepted: 6 April 2023

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.