Elusive AGN in theXMM-Newtonbright serendipitous survey

Abstract

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] 10 44 erg s −1 ). 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%). 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 ( 10 44 erg s −1 ) it is due to the optical absorption (in the Compton-thin regime, i.e. NH < 10 24 cm −2 ). Alternative hypotheses, like the presence of Compton-thick sources, BL Lac objects or “non-standard” AGN (e.g. with αOX < 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.