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The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program thathas imaged the central 0.9 deg^2 of the COSMOS field down to limiting depths of1.9 10^-16 erg cm^-2 s-1 in the 0.5-2 keV band, 7.3 10^-16 erg cm^-2 s^-1 inthe 2-10 keV band, and 5.7 10^-16 erg cm^-2 s-1 in the 0.5-10 keV band. In thispaper we report the i, K and 3.6micron identifications of the 1761 X-ray pointsources. We use the likelihood ratio technique to derive the association ofoptical/infrared counterparts for 97% of the X-ray sources. For most of theremaining 3%, the presence of multiple counterparts or the faintness of thepossible counterpart prevented a unique association. For only 10 X-ray sourceswe were not able to associate a counterpart, mostly due to the presence of avery bright field source close by. Only 2 sources are truly empty fields.Making use of the large number of X-ray sources, we update the "classic locus"of AGN and define a new locus containing 90% of the AGN in the survey with fullband luminosity >10^42 erg/s. We present the linear fit between the total iband magnitude and the X-ray flux in the soft and hard band, drawn over 2orders of magnitude in X-ray flux, obtained using the combined C-COSMOS andXMM-COSMOS samples. We focus on the X-ray to optical flux ratio (X/O) and wetest its known correlation with redshift and luminosity, and a recentlyintroduced anti-correlation with the concentration index (C). We find a stronganti-correlation (though the dispersion is of the order of 0.5 dex) between Cand X/O, computed in the hard band, and that 90% of the obscured AGN in thesample with morphological information live in galaxies with regular morphology(bulgy and disky/spiral), suggesting that secular processes govern asignificant fraction of the BH growth at X-ray luminosities of 10^43- 10^44.5erg/s.

The Chandra COSMOS Survey: III. Optical and Infrared Identification of X-ray Point Sources