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Elusive AGN in the XMM-Newton bright serendipitous survey

AuthorsCaccianiga, A.; Severgnini, P.; Della Ceca, R.; Maccacaro, T.; Carrera, Francisco J. CSIC ORCID CVN ; Page, M. J.
X-ray: galaxies
Galaxies: nuclei
Galaxies: active
Issue Date2007
PublisherEDP Sciences
CitationAstronomy and Astrophysics 470(2): 557-570 (2007)
Abstract[Context]: Optical follow up of X-ray selected sources finds a significant fraction ot >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 (∼1.5% of elusive AGN with L[2-10 keV] > 1044 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 (<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 < 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.
Publisher version (URL)
Identifiersdoi: 10.1051/0004-6361:20077732
issn: 0004-6361
e-issn: 1432-0746
Appears in Collections:(IFCA) Artículos

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