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 x 10(-15)-4 x 10(-13) erg cm(-2) s(-1) and the total area surveyed is similar to 0.5 deg(2) 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 similar to 1.4 deg(-2) 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 similar to 25. We measured the flux and luminosity of the [OIII]lambda 5007 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 L2-10 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 L2-10 keV/L-[OIII] greater than or similar to 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.