The Galactic plane at faint X-ray fluxes - II. Stacked X-ray spectra of a sample of serendipitous XMM-Newton sources

Abstract

We have investigated the X-ray spectral properties of a sample of 138 X-ray sources detected serendipitously in $XMM-Newton$ observations of the Galactic plane, at an intermediate to faint flux level. We divide our sample into 5 subgroups according to the spectral hardness of the sources, and stack (i.e. co-add) the individual source spectra within each subgroup. As expected these stacked spectra show a softening trend from the hardest to the softest subgroups, which is reflected in the inferred line-of-sight column density. The spectra of the three hardest subgroups are characterized by a hard continuum plus superimpose Fe-line emission in the 6--7 keV bandpass. The average equivalent width (EW) of the 6.7-keV He-like Fe-K$\alpha$ line is 170${}_{-32}^{+35}$ eV, whereas the 6.4-keV Fe-K fluorescence line from neutral iron and the 6.9-keV H-like Fe-Ly$\alpha$ line have EWs of 89${}_{-25}^{+26}$ eV and 81${}_{-29}^{+30}$ eV respectively, i.e. roughly half that of the 6.7-keV line. The remaining subgroups exhibit soft thermal spectra. Virtually all of the spectrally-soft X-ray sources can be associated with relatively nearby coronally-active late-type stars, which are evident as bright near-infrared (NIR) objects within the X-ray error circles. On a similar basis only a minority of the spectrally-hard X-ray sources have likely NIR identifications. The average continuum and Fe-line properties of the spectrally-hard sources are consistent with those of magnetic cataclysmic variables but the direct identification of large numbers of such systems in Galactic X-ray surveys, probing intermediate to faint flux levels, remains challenging.

14 pages, 9 figures, 4 tables, accepted for publication in MNRAS

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• Broad-band characteristics of seven new hard X-ray selected cataclysmic variables
• Low-luminosity X-ray sources and the Galactic ridge X-ray emission
• XMM-Newton observations of eleven intermediate polars and possible candidates
• V902 Monocerotis: a likely disc-accreting intermediate polar
• Timing analysis of a sample of five cataclysmic variable candidates observed by the XMM-Newton satellite
• HP Cet and Swift J0820.6-2805: two candidate intermediate polars observed by XMM-Newton
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• X-ray observations of the nova shell IPHASX J210204.7+471015