X-ray luminosities of optically selected cataclysmic variables and application to the Galactic ridge X-ray emission.

By studying Swift X-ray spectra of an optically selected, non-magnetic sample of nearby cataclysmic variables (CVs), we show that there is a population with X-ray luminosity much lower than accounted for in existing studies. We find an average 0.5–10.0 keV luminosity of $8\times 10^{29}{\,{\rm erg \,s^{-1}}}$ which is an order of magnitude lower than observed in previous samples. Looking at the co-added X-ray spectrum of 20 CVs, we show that the spectral properties of this optically selected, low X-ray luminosity sample – likely characteristic of the dominant population of CVs – resemble that of their brighter counterpart, as well as the X-ray emission originating in the Galactic ridge. It is argued that if the space density of CVs is greater than the current estimates, as it is indeed predicted by population synthesis models, then CVs can significantly contribute to the Galactic ridge emission. [ABSTRACT FROM AUTHOR]