The XMM-Newton Wide-Field Survey in the COSMOS Field. III. Optical Identification and Multiwavelength Properties of a Large Sample of X-Ray-Selected Sources

International audienceWe present the optical identification of a sample of 695 X-ray sources detected in the first 1.3 deg2 of the COSMOS XMM-Newton survey, down to a 0.5-2 keV (2-10 keV) limiting flux of ~10-15 erg cm-2 s-1 (~5×10-15 erg cm-2 s-1). In order to identify the correct optical counterparts and to assess the statistical significance of the X-ray-to-optical associations we have used the ``likelihood ratio technique.'' Here we present the identification method and its application to the CFHT I-band and photometric catalogs. We were able to associate a candidate optical counterpart to ~90% (626) of the X-ray sources, while for the remaining ~10% of the sources we were not able to provide a unique optical association due to the faintness of the possible optical counterparts (IAB>25) or to the presence of multiple optical sources, with similar likelihoods of being the correct identification, within the XMM-Newton error circles. We also cross-correlated the candidate optical counterparts with the Subaru multicolor and ACS catalogs and with the Magellan/IMACS, zCOSMOS, and literature spectroscopic data; the spectroscopic sample comprises 248 objects (~40% of the full sample). Our analysis of this statistically meaningful sample of X-ray sources reveals that for ~80% of the counterparts there is a very good agreement between the spectroscopic classification, the morphological parameters as derived from ACS data, and the optical-to-near-infrared colors: the large majority of spectroscopically identified broad-line active galactic nuclei (BL AGNs) have a pointlike morphology on ACS data, blue optical colors in color-color diagrams, and an X-ray-to-optical flux ratio typical of optically selected quasars. Conversely, sources classified as narrow line AGNs or normal galaxies are on average associated with extended optical sources, have significantly redder optical-to-near-infrared colors, and span a larger range of X-ray-to-optical flux ratios. However, about 20% of the sources show an apparent mismatch between the morphological and spectroscopic classifications. All the ``extended'' BL AGNs lie at redshift Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA; also based on data collected at the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under NASA contract NAS 5-26555 the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the European Southern Observatory, Chile, under Large Program 175.A-0839 Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, and the National Optical Astronomy Observatory, which are operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation; and the Canada-France-Hawaii Telescope operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France and the University of Hawaii