Abstract:
Cataclysmic variables are interacting binaries composed of a white dwarf that is accreting mass from a low-mass donor, evolving from longer to shorter orbital periods through angular momentum loss. Short-period cataclysmic variables with brown dwarf donors represent the final stage in the evolution of cataclysmic variables. Due to continuous mass loss, the donor in these short-period systems can not sustain hydrogen burning and becomes degenerate. The degeneracy of the donor reverses the orbital period evolution back towards longer periods, where such systems are, therefore, dubbed period-bouncers. According to population studies, the majority of cataclysmic variables, between 40 % and 80 %, are predicted to be period-bouncers, however, only 18 of these systems have been observed and confidently identified so far. The main focus of this thesis is to uncover the missing population of period-bouncers in order to provide observational support to the evolutionary theories of cataclysmic variables.
An X-ray detection of period-bouncers is a powerful tool in order to identify this underrepresented sub-type of cataclysmic variables, as it provides proof of the accretion in the system as the individual X-ray emission coming from either the white dwarf or the brown dwarf is below the sensitivity of current instruments. In order to identify the missing population of period-bouncer a three-step approach was implemented. First, a detailed X-ray study of a pilot-sample of magnetic period-bouncers was performed, leading to the first ever detection of period-bouncers using the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) onboard the Spektrum-Roentgen-Gamma spacecraft (SRG) reported by Muñoz-Giraldo et al. (2023). Hereby, specific X-ray characteristics of this elusive class of cataclysmic variables were established. Second, a literature catalog of known cataclysmic variables around the period-bounce was compiled, including the 18 previously known period-bouncers, in order to produce a “scorecard” reported by Muñoz-Giraldo et al. (2024) which assigns to each system the probability of being a period-bouncer. The X-ray data from the recent all-sky surveys carried out with eROSITA was systematically analysed to confirm 12 new period-bouncers, leading to a ∼66 % increase in the population of period-bouncers. After the confirmation of new period-bouncers from the last step this sub-type of cataclysmic variables still corresponds to only ∼11 % of the overall cataclysmic variable population. In the third step, a catalog of new period-bounce candidates was compiled from cool white dwarf candidates with an eROSITA detection that have a high likelihood of being a period-bouncer according to a “reduced” version of the scorecard. Confirmation from follow-up surveys of these period-bounce candidates could lead to the identification of the missing population of period-bouncers.
