Physics
Scientific paper
Oct 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agn8.confe..15d&link_type=abstract
Proceedings of the 8th National Conference on AGN, held in Torino, Italy, May 19-22, 2008. Edited by L. Lanteri, C.M. Raiteri,
Physics
Scientific paper
X-ray surveys have revealed a population of objects with very high X-ray-to-optical flux ratio (fx/fopt > 10), more than a factor 10 higher than typical X-ray selected AGN. The true nature of these extreme objects, called EXOs (Extreme X-ray-to-Optical ratio sources), is still not fully understood. A significant fraction of the EXO population has been found to exhibit very red optical-NIR colours (r-K > 5) and shows signatures of substantial absorption in X-rays. These objects are the best candidates to be the mostly elusive type 2 QSOs, required by synthesis models of the X-ray background. Here we present the multi-wavelength properties of one of the largest samples of bright EXOs assembled so far (130 sources with fx>10^-13 erg/cm^2/s, 0.2-12 keV), drawn from the cross-correlation between 2XMMp and SDSS-DR5 catalogues. Using detailed analysis of X-ray colours we are able to place limits on the size of the obscured population represented by our sample. UKIRT near-IR data, obtained for ~30% of our EXOs show evidence of substantial reddening for most of them, supporting the classification of type 2 AGN. However, optical spectra obtained for 12% of the sample reveal an unespected large fraction of type 1 QSOs amongst our EXOs.
Akiyama Masayuki
Del Moro Agnese
Mateos Silvia
Watson Michael G.
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