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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

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posted on 01.02.2016, 10:44 by Robert Seymour Warwick, Kristiina Byckling, D. Perez-Ramirez
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 five 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 superimposed Fe-line emission in the 6–7 keV bandpass. The average equivalent width (EW) of the 6.7-keV He-like Fe Kα line is 170+35−32−32+35 eV, whereas the 6.4-keV Fe K fluorescence line from neutral iron and the 6.9-keV H-like Fe Lyα line have EWs of 89+26−25−25+26 and 81+30−29−29+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.

History

Citation

Monthly Notices Of The Royal Astronomical Society, 2014, 438 (4), pp. 2967-2979 (13)

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy

Version

VoR (Version of Record)

Published in

Monthly Notices Of The Royal Astronomical Society

Publisher

Oxford University Press (OUP)

issn

0035-8711

eissn

1365-2966

Acceptance date

11/12/2013

Copyright date

2014

Available date

01/02/2016

Publisher version

http://mnras.oxfordjournals.org/content/438/4/2967

Language

en