The HELLAS2XMM survey. VIII. Optical identifications of the extended sample

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^-15-4 × 10^-13 erg cm^-2 s^-1 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 R∼25. We measured the flux and luminosity of the [OIII]λ5007 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 log N-log S. We find obscured QSOs surface density of 45 ± 15 and 100-350 deg^-2 down to flux limits of 10^-14 and 10^-15 erg cm^-2 s^-1, 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 L_{2-10 keV}/L_[OIII] 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 and/or 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 L_{2-10 keV}/L_[OIII]⪆ 1000, 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.

Based on observations collected at the European Southern Observatory, Prog. ID 67.A-0401, 68.A-0514, 69.A-0563 and 072.A-0633, and at the Telescopio Nazionale Galileo, Prog. ID 1_15_083. Based also on observations made with XMM-Newton, an ESA science mission. Tables 1, 3, 4 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/466/31