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

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_M = 1.37^{+0.3}_{-0.16} x 10^{-5} M_sun^{-1}) and luminosity density per solar mass (\rho_L = 8.95^{+0.15}_{-0.1} x 10^{26} erg s^{-1} M_sun^{-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 d\rho_M/d(\log_{10} L_x) and d\rho_L/d(\log_{10} L_x) confirms that there are two populations of hard X-ray emitting CVs. Intermediate polars dominate at luminosities L > 10^{33} 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.