The HELLAS2XMM 1dF survey: On the nature of high X-ray/optical flux sources

We present results from the photometric and spectroscopic identification of 122 X-ray sources recently discovered by XMM-Newton in the 2-10 keV band (the HELLAS2XMM 1 dF sample). One of the most interesting results (which is found also in deeper surveys) is that similar to 20% of the sources have an X-ray to optical flux ratio (X/O) ten times or more higher than that of optically selected AGN. Unlike the faint sources found in the ultra-deep Chandra and XMM-Newton surveys, which reach X-ray (and optical) fluxes 10 times lower than in the HELLAS2XMM sample, many of the extreme X/O sources in our sample have R less than or similar to 25 and are therefore accessible to optical spectroscopy. We report the identification of 13 sources with extreme X/O values. While four of these sources are broad line QSO, eight of them are narrow line QSO, seemingly the extension to very high luminosity of the type 2 Seyfert galaxies.