On the space density of X-ray selected cataclysmic variables

The space density of the various classes of cataclysmic variables (CVs) could only be weakly constrained in the past. Reasons were the small number of objects in complete X-ray flux-limited samples and the difficulty to derive precise distances to CVs. The former limitation still exists. Here the impact of Gaia parallaxes and implied distances on the space density of X-ray selected complete, flux-limited samples is studied. The samples are described in the literature, those of non-magnetic CVs are based on ROSAT (RBS - ROSAT Bright Survey & NEP -- North Ecliptic Pole), that of the Intermediate Polars stems from Swift/BAT. All CVs appear to be rarer than previously thought, although the new values are all within the errors of past studies. Upper limits at 90\% confidence for the space densities of non-magnetic CVs are $\rho_{\rm RBS} < 1.1 \times 10^{-6}$ pc$^{-3}$, and $\rho_{\rm RBS+NEP} < 5.1 \times 10^{-6}$ p$^{-3}$, for an assumed scale height of $h=260$ pc and $\rho_{\rm IPs} < 1.3 \times 10^{-7}$ p$^{-3}$ for the long-period Intermediate Polars at a scale height of 120 pc. Most of the distances to the IPs were under-estimated in the past. The upper limits to the space densities are only valid in the case where CVs do not have lower X-ray luminosities than the lowest-luminosity member of the sample. These results need consolidation by larger sample sizes, soon to be established through sensitive X-ray all-sky surveys to be performed with eROSITA on the Spektrum-X-Gamma mission.