Properties of the Galactic population of cataclysmic variables in hard X-rays

Abstract

We measure the spatial distribution and hard X-ray luminosity function of cataclysmic variables (CVs) using the INTEGRAL all-sky survey in the 17-60 keV energy band. The vast majority of CVs detected by INTEGRAL are intermediate polars of luminosity in the range 10³²-10³⁴ erg s^-1. The scale height of the Galactic disk population of intermediate polars is found to be 130^+93_-46 pc. The CV luminosity function measured with INTEGRAL in hard X-rays is compatible with that previously determined at lower energies (3-20 keV) using a largely independent sample of sources detected by RXTE (located at |b| > 10° as opposed to the INTEGRAL sample, which is strongly concentrated to the Galactic plane). The cumulative 17-60 keV luminosity density of CVs per unit stellar mass is found to be (1.3±0.3)×10²⁷ erg s^-1 M^-1_⊙, which is comparable to that of low-mass X-ray binaries in this energy band. Therefore, faint but numerous CVs are expected to provide an important contribution to the cumulative hard X-ray emission of galaxies.