2007A&A...470..557C

Optical follow-up of X-ray selected sources finds a significant fraction of ``optically dull'' sources characterized by optical spectra without obvious signature of AGN activity. In many cases, however, the presence of an AGN is inferred from other diagnostics (e.g. the X-ray properties). Understanding and accounting for this ``elusiveness'' is mandatory for a reliable study of the AGN physical and statistical properties. We investigate here the nature of all the sources (35 in total) in the XMM-Newton bright serendipitous survey (which is 86% optically identified) showing an optical spectrum dominated by the light from the host galaxy with no evidence (or little evidence) for the presence of an AGN. We use the X-ray spectral analysis to assess the presence of an AGN in these sources and to characterize its properties. We detect AGN activity in 33 out of 35 sources. The remaining 2 sources are the ones with the lowest X-ray luminosity in the sample (L_[2-10 keV]_<10⁴¹erg/s) and their X-ray emission could be produced within the host galaxy. We find that the ``recognition problem'' for AGN is very critical in the low-luminosity regime (at least 60% of the AGN with L_[2-10 keV]_<10<sup>43</sup>erg/s are elusive) becoming negligible for high X-ray luminosities (∼1.5% of elusive AGN with L_[2-10 keV]_>10<sup>44</sup>erg/s). This problem affects mostly absorbed AGN (∼40% of type 2 AGN in the survey are elusive) but also a significant fraction of unabsorbed AGN (8%). We find that the simplest explanations of why these 33 (or most of them) AGNs are elusive are two: at low X-ray luminosities (<10<sup>43</sup>erg/s) the most important reason is the intrinsically low AGN/galaxy contrast (optical dilution) while at high luminosities (>10<sup>44</sup>erg/s) it is due to the optical absorption (in the Compton-thin regime, i.e. N<sub>H</sub><10<sup>24</sup>cm<sup>–2</sup>). Alternative hypotheses, like the presence of Compton-thick sources, BL Lac objects or ``non-standard'' AGN (e.g. with α_OX<1 or with weak/absorbed Narrow Line Region) are not supported by the data although we cannot exclude the presence in the sample of a few sources of these types.