New X-ray detections of magnetic period-bounce cataclysmic variables from XMM-Newton and SRG/eROSITA

Context. A great portion of the cataclysmic variable population, between 40% and 70%, is predicted to be made up of period-bouncers, systems with degenerate donors that have evolved past the period minimum. However, due to their intrinsic faintness, only a few of these systems have been observed and confidently identified so far.
Aims: We have searched for X-ray emission as proof of accretion in order to confirm period-bounce cataclysmic variables.
Methods: In this study, we used data from XMM-Newton and eROSITA on a pilot sample of three period-bounce candidates with a magnetic white dwarf, which are expected to exhibit stronger X-ray emission than non-magnetic systems due to more efficient conversion of matter accretion onto the white dwarf.
Results: In a dedicated XMM-Newton observation of the period-bounce candidate SDSS J151415.65+074446.5 we discovered X-ray modulation at the binary orbital period confirming it as an accreting system. The X-ray light curve and the X-ray spectrum display characteristics of magnetic Polar-type systems, allowing us for the first time to determine the X-ray luminosity and mass accretion rate for this system. Catalog data from eROSITA on the Spektrum-Roentgen-Gamma satellite for V379 Vir and SDSS J125044.42+154957.4 enabled a first look into the X-ray behavior of period-bounce candidates with this new all-sky instrument. From the eROSITA measurements, the X-ray luminosity and mass accretion rate were determined for the first time for SDSS J125044.42+154957.4, and the earlier result for V379 Vir from XMM-Newton was confirmed.
Conclusions: The three cataclysmic variables with a magnetic white dwarf and very low-mass donor studied in this work present evidence for X-ray emission at a similar level of L_X [erg s⁻¹] ≈ 10²⁹, which, together with the detection of X-ray orbital modulation in two of them (i.e., V379 Vir and SDSS J151415.65+074446.5), unambiguously proves the presence of accretion in these systems. The detection of these period-bouncers at faint X-ray luminosity levels with the all-sky X-ray survey eROSITA offers new prospects for the identification of additional period-bouncers, providing impetus for theoretical studies of binary evolution.