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arXiv · 1808.08144

On the space density of X-ray selected cataclysmic variables

A. Schwope
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Abstract

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.

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A. Schwope. 2018-08-24. On the space density of X-ray selected cataclysmic variables. https://doi.org/10.1051/0004-6361%2F201833723

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