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 (M = 1.37+0.3-0.16 x 10-5 Msun-1) and luminosity density per solar mass (L = 8.95+0.15-0.1 x 1026 erg s-1 Msun-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 dM/d(10 Lx) and dL/d(10 Lx) 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.