X-ray and ultraviolet observations of the eclipsing cataclysmic variables OV Bootis and SDSS J103533.02+055158.3 with degenerate donors

Abstract

Context.The majority of cataclysmic variables are predicted to be post-period minimum systems with degenerate donor stars, the period bouncers. Owing to their intrinsic faintness, however, only a handful of these systems have so far been securely identified.Aims.We want to study the X-ray properties of two eclipsing period bouncers, OV Bootis and SDSS J103533.02+055158.3, that were selected for this study due to their proximity to Earth.Methods.We have obtainedXMM-Newtonphase-resolved X-ray and ultraviolet observations of the two objects for spectral and timing analysis.Results.Owing to a recent dwarf nova outburst OV Bootis was much brighter than SDSS J103533.02+055158.3 at X-ray and ultraviolet wavelengths and the eclipse could be studied in some detail. An updated eclipse ephemeris was derived. The X-rays were shown to originate close to the white dwarf, the boundary layer, with significant absorption affecting its spectrum. There was no absorption in SDSS J103533.02+055158.3, despite being observed at the same inclination indicating different shapes of the disk and the disk rim. The white-dwarf temperature was re-determined for both objects: the white dwarf in OV Bootis was still hot (23 000 K) five months after a dwarf nova outburst, and the white dwarf in SDSS J103533.02+055158.3 hotter than assumed previously (Teff = 11 500 K).Conclusions.All three cataclysmic variables with degenerate donors studied so far in X-rays, including SDSS J121209.31+013627.7, were clearly discovered in X-rays and revealed mass accretion ratesṀ= 8 × 10−15− 4 × 10 − 13M⊙yr−1. If their X-ray behavior is representative of the subpopulation of period bouncers, the all-sky X-ray surveys with eROSITA together with comprehensive follow-up will uncover new objects in sufficient number to address the remaining questions concerning late-stage cataclysmic variable evolution.