The Chandra COSMOS Legacy survey: optical/IR identifications
  We present the catalog of optical and infrared counterparts of the Chandra
COSMOS-Legacy Survey, a 4.6 Ms Chandra program on the 2.2 square degrees of the
COSMOS field, combination of 56 new overlapping observations obtained in Cycle
14 with the previous C-COSMOS survey. In this Paper we report the i, K, and 3.6
micron identifications of the 2273 X-ray point sources detected in the new
Cycle 14 observations. We use the likelihood ratio technique to derive the
association of optical/infrared (IR) counterparts for 97% of the X-ray sources.
We also update the information for the 1743 sources detected in C-COSMOS, using
new K and 3.6 micron information not available when the C-COSMOS analysis was
performed. The final catalog contains 4016 X-ray sources, 97% of which have an
optical/IR counterpart and a photometric redshift, while 54% of the sources
have a spectroscopic redshift. The full catalog, including spectroscopic and
photometric redshifts and optical and X-ray properties described here in
detail, is available online. We study several X-ray to optical (X/O)
properties: with our large statistics we put better constraints on the X/O flux
ratio locus, finding a shift towards faint optical magnitudes in both soft and
hard X-ray band. We confirm the existence of a correlation between X/O and the
the 2-10 keV luminosity for Type 2 sources. We extend to low luminosities the
analysis of the correlation between the fraction of obscured AGN and the hard
band luminosity, finding a different behavior between the optically and X-ray
classified obscured fraction.
