Hard X-ray luminosity functions of cataclysmic variables: joint Swift/BAT and Gaia data

Abstract

ABSTRACT Cataclysmic variables (CVs) are the most numerous population among the Galactic objects emitting in hard X-rays. Most probably, they are responsible for the extended hard X-ray emission of the Galactic ridge and the central Galactic regions. Here, we consider the sample of CVs detected in the all-sky hard X-ray Swift/BAT survey, which were also detected by Gaia and thus have reliable distance estimates. Using these data, we derive accurate estimates for local number density per solar mass ($\rho _{\rm M} = 1.37^{+0.3}_{-0.16}. \times 10^{-5}\, {\rm M}_\odot ^{-1}$) and luminosity density per solar mass ($\rho _{\rm L} = 8.95^{+0.15}_{-0.1}\times 10^{26}$ erg s−1 M$_\odot ^{-1}$) for objects in the sample. These values appear to be in good agreement with the integrated Galactic ridge X-ray emission and nuclear stellar cluster luminosities. Analysis of the differential luminosity functions dρM/d(log10Lx) and dρL/d(log10Lx) confirms that there are two populations of hard X-ray-emitting CVs. Intermediate polars dominate at luminosities L > 1033 erg s−1, whereas non-magnetic CVs and polars are much more numerous but have lower luminosities on average. As a consequence, the contribution of these populations to the observed hard X-ray luminosity is almost equivalent.