Elusive AGN in the XMM-Newton bright serendipitous survey

Context. 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. Aims. 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. Methods. We use the X-ray spectral analysis to assess the presence of an AGN in these sources and to characterize its properties. Results. 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] < 1041 erg s-1) 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] < 1043 erg s-1 are elusive) becoming negligible for high X-ray luminosities (similar to 1.5% of elusive AGN with L[2-10 keV] > 1044 erg s-1). This problem a. ects mostly absorbed AGN (similar to 40% of type 2 AGN in the survey are elusive) but also a significant fraction of unabsorbed AGN (8%). Conclusions. We find that the simplest explanations of why these 33 (or most of them) AGNs are elusive are two: at low X-ray luminosities (< 1043 erg s(-1)) the most important reason is the intrinsically low AGN/ galaxy contrast (optical dilution) while at high luminosities (> 1044 erg s(-1)) it is due to the optical absorption (in the Compton-thin regime, i. e. NH < 1024 cm-2). Alternative hypotheses, like the presence of Compton-thick sources, BL Lac objects or non-standard AGN (e. g. with alpha(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.