The HELLAS2XMM survey^*,**

VIII. Optical identifications of the extended sample

¹ INAF – Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio-Catone (RM) 00040, Italy e-mail: cocchia@brera.mi.astro.it
² Dipartimento di Fisica, Università di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Rome, Italy
³ INAF – Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
⁴ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁶ Max Planck Institüt für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany
⁷ Harvard-Smithsonian Center for Astrophysics (CfA), 60 Garden str, Cambridge 02138 MA, USA
⁸ INAF – IASF, via Bassini 15, 20133 Milano, Italy
⁹ Dipartimento di Fisica, Università Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
¹⁰ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
¹¹ ASI Science Data Center, ASDC c/o ESRIN, via G. Galilei, 00044 Frascati, Italy

Received: 9 March 2006
Accepted: 1 December 2006

Abstract

Aims.Hard X-ray, large-area surveys are a fundamental complement to ultra-deep, pencil-beam surveys in obtaining more complete coverage of the AGN luminosity-redshift plane and finding sizeable samples of “rare” AGN.

Methods.We present the results of the photometric and spectroscopic identification of 110 hard X-ray selected sources from 5 additional XMM-Newton fields, nearly doubling the original HELLAS2XMM sample. Their 2–10 keV fluxes cover the range 6    10^-13  and the total area surveyed is ~0.5 deg² at the bright flux limit. We spectroscopically identified 59 new sources, bringing the spectroscopic completeness of the full HELLAS2XMM sample to almost 70% over a total area of ~1.4 deg² at the bright flux limit. We found optical counterparts for 214 out of the 232 X-ray sources of the full sample down to . We measured the flux and luminosity of the [OIII] emission line for 59 of these sources.

Results.Assuming that most high X-ray-to-optical flux ratio sources are obscured QSOs, we used the full HELLAS2XMM sample and the CDF samples to estimate their . We find obscured QSOs surface density of 45 ± 15 and 100–350 deg^-2 down to flux limits of 10^-14 and 10^-15 , respectively. At these flux limits, the fraction of X-ray-selected obscured QSOs turns out to be similar to that of unobscured QSOs. Since X-ray selection misses most Compton-thick AGN, the number of obscured QSOs may well outnumber the unobscured QSOs. 
We find that hard X-ray selected AGNs with a detected [OIII] emission span a wide range of with a logarithmic median of 2.14 and interquartile range of 0.38. This is marginally higher than for a sample of optically selected AGNs (median 1.69 and interquatile range 0.30), suggesting that optically selected samples are at least partly incomplete andor that [OIII] emission is not a perfect isotropic indicator of the nuclear power. The seven X-ray bright, optically normal galaxy (XBONG) candidates in the sample have , while their X-ray and optical luminosities and obscuring column density are similar to those of narrow-line AGNs in the same redshift interval (0.075–0.32). This suggests that, while the central engine of narrow-line AGNs and XBONGs looks similar, the narrow-line region in XBONGs could be strongly inhibited or obscured.