Abstract
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^(28-34) erg/s (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 x 10^28 erg/s/M and 2.5 +/- 0.6 x 10^27 erg/s/M, for the ASBs and CVs respectively, which in total is a factor 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 deg 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 roughly 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.
Highlights
The hard Galactic ridge X-ray emission (GRXE) can be traced from the Galactic Centre, where its surface brightness peaks, along the Galactic plane out to |l| = 60◦ and beyond (Worrall et al 1982; Warwick et al 1985; Koyama et al 1986a; Yamauchi & Koyama 1993; Yamauchi et al 1996; Revnivtsev et al 2006; Koyama et al 2007)
Using the spectral forms previous defined for the two source populations, we find that the fraction of the 2–10 keV flux contained within this narrower band is 0.21 and 0.39 for the ASBs and cataclysmic variables (CVs), respectively
Applying these factors and assuming that the Galactic absorption in the 6–10 keV band is small, we find that the ASBs and CVs contribute 62 ± 10 and 21 ± 5 per cent of the 6–10 keV GRXE
Summary
We discuss the properties of a sample of lowluminosity Galactic X-ray sources detected in the XSS in the hard X-ray band. This sample comprises coronally active stellar sources plus a smaller number of CVs (magnetic and non-magnetic). We discuss various aspects of our results in the context of the origin of the GRXE (Section 6) and summarize our conclusions (Section 7)
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