MNRAS-2004-Georgakakis-135-45.pdf (478.08 kB)
The XMM-Newton/2dF survey - II. The nature of X-ray - faint optically bright X-ray sources
journal contributionposted on 2009-12-08, 16:24 authored by A. Georgakakis, I. Georgantopoulos, M. Vallbe, V. Kolokotronis, S. Basilakos, S. M. Plionis, G. C. Stewart, T. Shanks, B. J. Boyle
In this paper we investigate the properties of low X-ray-to-optical flux ratio sources detected in a wide-area (2.5 deg2) shallow [fX(0.5–8 keV) ≈ 10−14 erg s−1 cm−2]XMM–Newton survey. We find a total of 26 sources (5 per cent of the total X-ray-selected population) with log fX/fopt < −0.9 to the above flux limit. Optical spectroscopy is available for 20 of these low X-ray-to-optical flux ratio objects. Most of them are found to be associated with Galactic stars (a total of eight) and broad-line active galactic nuclei (AGNs; a total of eight). We also find two sources with optical spectra showing absorption and/or narrow emission lines and X-ray/optical properties suggesting AGN activity. Another two sources are found to be associated with low-redshift galaxies with narrow emission-line optical spectra, X-ray luminosities LX(0.5–8 keV) ≈ 1041 erg s−1 and log fX/fopt≈−2 suggesting ‘normal’ star-forming galaxies. Despite the small-number statistics the sky density of ‘normal’ X-ray-selected star-forming galaxies at the flux limit of the present sample is low, consistent with previous ROSAT High-Resolution Imager (HRI) deep surveys. Also, the number density estimated here is in good agreement with both the log N–log S of ‘normal’ galaxies in the Chandra Deep Field North (extrapolated to bright fluxes) and model predictions based on the X-ray luminosity function of local star-forming galaxies.