The XMM-Newton/2dF survey - II. The nature of X-ray-faint optically bright X-ray sources

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 [f_X(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 f_X/f_opt < -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 L_X(0.5-8 keV) ~ 10⁴¹ erg s^-1 and log f_X/f_opt~-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.