The Chandra COSMOS Legacy survey: optical/IR identifications

TL;DR

This paper presents a comprehensive catalog of optical and infrared counterparts for X-ray sources in the Chandra COSMOS Legacy Survey, enhancing source identification and redshift information for over 4000 objects.

Contribution

The study provides an updated, extensive catalog of X-ray sources with optical/IR identifications, redshifts, and properties, improving previous data with new observations and refined association techniques.

Findings

97% of X-ray sources have optical/IR counterparts.

Confirmed correlation between X/O ratio and 2-10 keV luminosity for Type 2 AGN.

Extended analysis of obscured AGN fraction at low luminosities.

Abstract

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.