SIMBAD references

Cataclysmic variables (CVs) represent a key evolutionary phase of many low-mass stellar binaries and appear to collectively dominate the galactic X-ray background above ∼1 keV. Statistical properties of CVs, however, remain poorly understood, even in the solar neighbourhood. We aim to obtain an X-ray luminosity-complete sample of local CVs whose distances put them within three volumes. Here we present a pilot investigation on the X-ray and optical properties of local CVs or their candidates, mainly by cross-correlating the 3XMM DR8 and Gaia DR2 source catalogues. We first examine a nearly complete sample of CVs within 150 pc by characterizing their X-ray luminosity, X-ray to Gaia G-band flux ratio, and X-ray hardness ratio distributions, as well as their locations in the Gaia colour-magnitude diagram. We show that these distributions of this very local sample are consistent with those of a CV sample obtained at distances between 150 and 300 pc. We also present a catalogue of 15 new CV candidates within 500 pc based primarily on their X-ray to Gaia G-band flux ratios. The X-ray luminosities of these candidates are between 10²⁹ to several 10³¹ erg s^–1, and their stacked X-ray spectrum can be well described by a multitemperature optically thin thermal plasma model with a maximum temperature $T_{\rm max}=7.5^{+6.3}_{-2.4}$ keV. These properties are consistent with those of the very local CVs. In particular, one of the CV candidates is within 150 pc, indicating that a considerable number of local CVs are yet detected. These results provide a base for a more complete statistical understanding of CVs and their quantitative contribution to the galactic X-ray background.