Serendipitous discovery of the magnetic cataclysmic variable SRGE J075818-612027

Abstract

We report the discovery of SRGE J075818-612027, a deep stream-eclipsing magnetic cataclysmic variable found serendipitously in SRG/eROSITA CalPV observations of the open cluster NGC~2516 as an unrelated X-ray source. An X-ray timing and spectral analysis of the eROSITA data is presented and supplemented by an analysis of TESS photometry and SALT spectroscopy. X-ray photometry reveals two pronounced dips repeating with a period of $106.144(1)$ min. The 14-month TESS data reveal the same unique period. A low-resolution identification spectrum obtained with SALT displays hydrogen Balmer emission lines on a fairly blue continuum. The spectrum and the stability of the photometric signal led to the classification of the new object as a polar-type cataclysmic variable. In this picture, the dips in the X-ray light curve are explained by absorption in the intervening accretion stream and by a self-eclipse of the main accretion region. The object displays large magnitude differences on long (months) timescales both at optical and X-ray wavelengths, being interpreted as high and low states and thus supporting the identification as a polar. The bright phase X-ray spectrum can be reflected with single temperature thermal emission with 9.7 keV and bolometric X-ray luminosity $L_{\mathrm{X}}\simeq 8\times 10^{32}$erg s${}^{-1}$ at a distance of about 2.7 kpc. It lacks the pronounced soft X-ray emission component prominently found in ROSAT-discovered polars.

9 Pages, 10 figures, accepted to A&A, 24 Feb 2023

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• Compact white-dwarf binaries in the combined SRG/eROSITA/SDSS eFEDS survey
• The eROSITA DR1 variability catalogue
• Confirmation of SRGt 062340.2-265751 as a nova-like cataclysmic variable with a possible magnetic nature