Exploring the space density of X-ray selected cataclysmic variables
Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
Accepted: 21 August 2018
The space density of the various classes of cataclysmic variables (CVs) has up to now only been weakly constrained, due to the small number of objects in complete X-ray flux-limited samples and the difficulty in deriving 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. These samples have been described in the literature: Those of non-magnetic CVs are based on ROSAT (RBS – ROSAT Bright Survey & NEP – North Ecliptic Pole) and that of the intermediate polars (IPs) 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 ρRBS < 1.1 × 10−6 pc−3 and ρRBS+NEP < 5.1 × 10−6 pc−3 for an assumed scale height of h = 260 pc and ρIPs < 1.3 × 10−7 pc−3 for the long-period IPs at a scale height of 120 pc. Most of the distances to the IPs have previously been under-estimated. The upper limits to the space densities are only valid in cases where CVs do not have lower X-ray luminosities than the lowest-luminosity member of the sample. These results require confirmation using 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.
Key words: novae, cataclysmic variables / surveys / X-rays: binaries
© ESO 2018