In this paper we investigate the properties of low X-ray–to–optical flux ratio sources detected in a wide area ( 2.5 deg ^ { 2 } ) shallow [ f _ { X } ( 0.5 - 8 keV ) \approx 10 ^ { -14 } erg s ^ { -1 } cm ^ { -2 } ] XMM- Newton survey .
We find a total of 26 sources ( 5 \% 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 ( total of 8 ) and broad line AGNs ( total of 8 ) .
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 ) \approx 10 ^ { 41 } erg s ^ { -1 } and \log f _ { X } / f _ { opt } \approx - 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 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 .