Cataclysmic variables around the period-bounce: An eROSITA-enhanced multiwavelength catalog

Abstract

Context. Cataclysmic variables (CVs) with degenerate donors that have evolved past the period minimum are predicted to make up a great portion of the CV population, namely, between 40% and 80%. However, either due to shortcomings in the models or the intrinsic faintness of these strongly evolved systems, only a few of these so-called “period-bouncers” have been confidently identified thus far. Aims. We compiled a multiwavelength catalog of period-bouncers and CVs around the period minimum from the literature to provide an in-depth characterization of the elusive subclass of period-bounce CVs that will support the identification of new candidates. Methods. We combined recently published or archival multiwavelength data with new X-ray observations from the all-sky surveys carried out with the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spektrum-Roentgen-Gamma spacecraft (SRG). Our catalog comprises 192 CVs around the period minimum, chosen as likely period-bounce candidates based on reported short orbital periods and low donor mass. This sample helped us establish specific selection parameters, which were used to compile a “scorecard” that rates the likelihood that a particular system is a period-bouncer. Results. Our “scorecard” correctly assigns high scores to the already confirmed period-bouncers in our literature catalog. It has also identified 103 additional strong period-bounce candidates in the literature that had not previously been classified as such. We established two selection cuts based on the X-ray-to-optical flux ratio (−1.21 ≤ log(Fx/Fopt) ≤ 0) and the typical X-ray luminosity (log(Lx,bol) ≤ 30.4 [erg s−1]) observed from the eight period-bouncers that have already been confirmed with eROSITA data. These X-ray selection cuts led to the updated categorization of seven systems as new period-bouncers, increasing their known population to 24 systems in total. Conclusions. Our multiwavelength catalog of CVs around the period minimum drawn from the literature, together with X-ray data from eROSITA, has resulted in a ~40% increase in the population of period-bouncers. Both the catalog and “scorecard” we constructed will aid in future searches for new period-bounce candidates. These tools will contribute to the goal of resolving the discrepancy between the predicted high number of period-bouncers and the low number of these systems successfully observed to date.