A first systematic characterization of cataclysmic variables in SRG/eROSITA surveys

Abstract

We present an account of known cataclysmic variables (CVs) that were detected as X-ray sources in eROSITA X-ray surveys and have gai DR3 counterparts. We address standard CVs with main sequence donors and white dwarfs accreting via Roche-lobe overflow (RLOF) and related objects, the double degenerates (DDs), and the symbiotic stars (SySts). We discern between nonmagnetic (dwarf novae and nova-like objects) and magnetic CVs (polars and intermediate polars (IPs)). In the publically available eROSITA catalog from the recent DR1, typically 65<!PCT!> of known cataloged and classified CVs are detected. This fraction rises to over 90<!PCT!> if the stack of all X-ray surveys (called S45 in this paper) is considered and the search volume is restricted to a radius of 500\,pc. We examine the various classes of CVs in various diagnostic diagrams relating X-ray and optical properties (luminosity, absolute magnitude, color, X-ray spectral hardness, and optical variability) and establish their average class properties. We derive spectral properties for the 22 brightest polars and confirm an increase in the ratio of soft to hard X-rays with increasing magnetic field in the accretion region. We report three new soft IPs and present a spectral analysis of all soft IPs. Their blackbody temperatures agree well with published values. The DDs represent the bluest and faintest subcategory but reach the same identification fraction as the standard CVs. The SySts are the most distant systems; only 20 (13<!PCT!>) were detected as X-ray sources in S45, and 7 of those are first-time detections. We investigate their mean properties using an upper limit on the flux of the nondetected CVs. Their X-ray nondetection is indeed a distance effect. We used all properties combined to select candidate CVs for all-sky optical identification programs, with the ultimate aim being to compose large CV samples in order to better constrain the impact of magnetic fields on the evolution of CVs, to derive space densities and luminosity functions, and to quantify the contribution of white-dwarf accreting systems to the Galactic Ridge X-ray emission (GRXE). The results of the optical identification program will be presented in forthcoming papers.