Optical identification of XMM sources in the Canada-France-Hawaii telescope legacy survey

Abstract

We present optical spectroscopic identifications of X-ray sources in ∼3 deg² of the XMM-Large Scale Structure survey (XMM-LSS), also covered by the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS), obtained with the AAOmega instrument at the Anglo-Australian Telescope. In a flux-limited sample of 829 point-like sources in the optical band with g^′≤ 22 mag and the 0.5-2 keV flux (f_{0.5-2 keV}) < 1 × 10^-15 erg cm^-2 s^-1, we observed 693 objects and obtained reliable spectroscopic identification for 487 sources, approximately 59 per cent of the overall sample. We therefore increase the number of identifications in this field by a factor close to 5. Galactic stellar sources represent about 15 per cent of the total (74/487). About 54 per cent (265/487) are broad-line active galactic nuclei (AGN) spanning redshifts between 0.15 and 3.87 with a median value of 1.68. The optical-to-X-ray spectral index (α_ox) of the broad-line AGN is 1.47 ± 0.03, typical of optically selected type I quasars, and is found to correlate with the rest-frame X-ray and optical monochromatic luminosities at 2 keV and 2500 Å, respectively. Consistent with previous studies, we find α_ox not to be correlated with z. In addition, 32 and 116 X-ray sources are, respectively, absorption- and emission-line galaxies at z > 0.76. From a line ratio diagnostic diagram, it is found that in about 50 per cent of these emission-line galaxies, the emission lines are powered significantly by the AGN. 30 of the XMM sources are detected at one or more radio frequencies. In addition, 24 sources have ambiguous identification: in eight cases, two XMM sources have a single optical source within 6 arcsec of each of them, whereas two and 14 XMM sources have, respectively, three and two possible optical sources within 6 arcsec of each of them. Spectra of multiple possible counterparts were obtained in such ambiguous cases.