The HELLAS2XMM 1dF survey: on the nature of high X-ray to optical flux ratio sources
F. Fiore, M. Brusa, F. Cocchia, A. Baldi, N. Carangelo, P. Ciliegi, A. Comastri, F. La Franca, R. Maiolino, G. Matt, S. Molendi, M. Mignoli, G.C. Perola, P. Severgnini, C. Vignali
Abstract
We present results from the photometric and spectroscopic identification of 122 X-ray sources discovered by XMM-Newton in the 2-10 keV band (the HELLAS2XMM 1dF sample). One of the most interesting results (which is found also in deeper sourveys) is that ~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~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.