Volume 548, December 2012
|Number of page(s)||15|
|Published online||29 November 2012|
Dept. of Physics and Astronomy, University of Leicester,
2 XMM SOC, ESAC, Apartado 78, 28691 Villanueva de la Cañada, Madrid, Spain
Received: 14 December 2011
Accepted: 13 October 2012
Context. The on-going XMM-Newton Slew Survey (XSS) provides coverage of a significant fraction of the sky in a broad X-ray bandpass. Although shallow by contemporary standards, in the “classical” 2–10 keV band of X-ray astronomy, the XSS provides significantly better sensitivity than any currently available all-sky survey.
Aims. We investigate the source content of the XSS, focussing on detections in the hard 2–10 keV band down to a very low threshold (≥ 4 counts net of background). At the faint end, the survey reaches a flux sensitivity of roughly 3 × 10-12 erg cm-2 s-1 (2–10 keV).
Methods. Our starting point was a sample of 487 sources detected in the XSS (up to and including release XMMSL1d2) at high galactic latitude in the hard band. Through cross-correlation with published source catalogues from surveys spanning the electromagnetic spectrum from radio through to gamma-rays, we find that 45% of the sources have likely identifications with normal/active galaxies. A further 18% are associated with other classes of X-ray object (nearby coronally active stars, accreting binaries, clusters of galaxies), leaving 37% of the XSS sources with no current identification. We go on to define an XSS extragalactic sample comprised of 219 galaxies and active galaxies selected in the XSS hard band. We investigate the properties of this extragalactic sample including its X-ray log N − log S distribution.
Results. We find that in the low-count limit, the XSS is, as expected, strongly affected by Eddington bias. There is also a very strong bias in the XSS against the detection of extended sources, most notably clusters of galaxies. A significant fraction of the detections at and around the low-count limit may be spurious. Nevertheless, it is possible to use the XSS to extract a reasonably robust sample of extragalactic sources, excluding galaxy clusters. The differential log N − log S relation of these extragalactic sources matches very well to the HEAO-1 A2 all-sky survey measurements at bright fluxes and to the 2XMM source counts at the faint end.
Conclusions. The substantial sky coverage afforded by the XSS makes this survey a valuable resource for studying X-ray bright source samples, including those selected specifically in the hard 2–10 keV band.
Key words: surveys / X-rays: general / galaxies: active
Appendix A is available in electronic form at http://www.aanda.org
Table A.1 is also available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (188.8.131.52) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/548/A99
© ESO, 2012
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