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

TL;DR

This study presents optical identifications and analyses of 110 hard X-ray sources from the HELLAS2XMM survey, revealing insights into obscured and unobscured QSOs, their luminosities, and the nature of XBONGs, with implications for AGN population studies.

Contribution

It provides the first comprehensive optical identification of a large X-ray selected sample, including new spectroscopic data and analysis of obscured and unobscured QSOs, and examines the properties of XBONGs.

Findings

Obscured and unobscured QSOs are similarly prevalent at certain flux levels.

Optically selected AGNs may be incomplete due to obscuration or anisotropic emission.

XBONGs have high L(2-10 keV)/L[OIII] ratios, indicating possible obscuration of the narrow-line region.

Abstract

(Abridged) We present the results of the photometric and spectroscopic identification of 110 hard X-ray selected sources (6e-15<F(2-10 keV)<4e-14 cgs) from 5 additional XMM-Newton fields, nearly doubling the original HELLAS2XMM sample. We spectroscopically identified 59 new sources, bringing the completeness of the full HELLAS2XMM sample to ~70%. We found optical counterparts for 214 out of the 232 X-ray sources of the full sample down to R~25. We measure the flux and luminosity of the [OIII](5007) emission line for 59 such sources. Results. We use the full HELLAS2XMM and the CDF samples to estimate the LogN-LogS and the obscured QSO surface densities down to F(2-10 keV)~1e-14 and 1e-15 cgs. At these flux limits the fraction of X-ray selected obscured and unobscured QSO turns out to be similar. Since X-ray selection misses most Compton thick AGN, the number of obscured QSO may well outnumber that of unobscured QSOs. We find that hard X-ray selected AGNs with detected [OIII] emission span a wide range of L(2-10 keV)/L[OIII] with a median higher than that of optically selected AGNs, suggesting that optically selected samples are at least partly incomplete, and/or [OIII] emission is not a perfect isotropic indicator of the nuclear power. Differently from narrow-line AGNs in the same z interval, the 7 XBONGs candidates in the sample have L(2-10 keV)/L[OIII]> 1000, while their Lx, L(R) and Nh are similar. 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.