Physics
Scientific paper
Oct 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agn8.confe..19l&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
Recent results indicate that selection criteria based on IR colors can be used to build large samples of obtically-obscured quasars candidate at z~1. We report an X-ray spectroscopic study of 44 bright (F24> 1.3 mJy) SWIRE sources with extreme mid-IR/Optical flux ratios (MIR/O>2000). The aim of this project is to provide a direct measurement of the column density of the absorber. The X-ray coverage of the sources in the sample is highly inhomogeneous. It includes both shallow Chandra observations (5 ks) and medium-deep XMM exposures (70 ks). We have spectroscopic redshift for ~20% of the sources (0.22<2.54), while for the remaining ones we are able to derive a photometric redshift (0.54<3.77) thanks to excellent optical and IR photometric coverage. For 55% of sources we have constrained the intrinsic Nh value, via the characterization of their X-ray spectral properties. 95% of them are found to be obscured AGN (Nh>10^22 cm^-2) and, remarkably, half of them show column density larger than 10^23 cm^-2. We also find that 50% of sources can be classified as type 2 QSOs on the basis of their Nh and X-ray luminosity (i.e. Nh>10^22 cm^-2 and X-ray luminosity>10^44 erg s^-1). We have therefore confirmed that the MIR/O>2000 selection applied to a large area survey like SWIRE is particularly efficient in discovering a large number of QSO2s a z>~1, that are traditionally missed in optical/soft X-ray surveys.
Feruglio Chiara
Fiore Fabrizio
Lanzuisi Giorgio
Piconcelli Enrico
Vignali Christian
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