Low-luminosity X-ray sources and the Galactic ridge X-ray emission

Using the XMM–Newton slew survey, we construct a hard-band selected sample of low-luminosity Galactic X-ray sources. Two source populations are represented, namely coronally active stars and binaries (ASBs) and cataclysmic variables (CVs), with X-ray luminosities collectively spanning the range 10[Superscript: 28–34] erg s[Superscript: −1] (2–10 keV). We derive the 2–10 keV X-ray luminosity function (XLF) and volume emissivity of each population. Scaled to the local stellar mass density, the latter is found to be 1.08 ± 0.16 × 10[Superscript: 28] and 2.5±0.6×10[Superscript: 27]ergs[Superscript: −1] M[Superscript: −1]⊙2.5±0.6×1027ergs−1 M⊙−1, for the ASBs and CVs, respectively, which in total is a factor of 2 higher than previous estimates. We employ the new XLFs to predict the X-ray source counts on the Galactic plane at l = 28 ∘..∘5 and show that the result is consistent with current observational constraints. The X-ray emission of faint, unresolved ASBs and CVs can account for a substantial fraction of the Galactic ridge X-ray emission (GRXE). We discuss a model in which ∼80 per cent of the 6–10 keV GRXE intensity is produced in this way, with the remainder attributable to X-ray scattering in the interstellar medium and/or young Galactic source populations. Much of the hard X-ray emission attributed to the ASBs is likely to be produced during flaring episodes.