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The Galactic plane at faint X-ray fluxes - II. Stacked X-ray spectra of a sample of serendipitous XMM-Newton sources
We have investigated the X-ray spectral properties of a sample of 138 X-ray
sources detected serendipitously in 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 line is
170 eV, whereas the 6.4-keV Fe-K fluorescence line from neutral
iron and the 6.9-keV H-like Fe-Ly line have EWs of 89 eV
and 81 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.Comment: 14 pages, 9 figures, 4 tables, accepted for publication in MNRA
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