The XMM-Newton survey of the ELAIS-S1 field II : optical identifications and multiwavelength catalogue of X-ray sources
Feruglio, C ; Fiore, F ; Franca, F La ; et al. ; - ASI Sponsor
Jan - 2008

journal :
type: Article Journal

Abstract
We present the optical identi cations and a multi-band catalogue of a sample of 478 X-ray sources detected in the XMM\Newton and Chandra surveys of the central 0.6 deg 2 of the ELAIS-S1 eld. The most likely optical/infrared counterpart of each X-ray source was identi ed using the chance coincidence probability in the R and IRAC 3.6 m bands. This method was complemented by the precise positions obtained through Chandra ob- servations. We were able to associate a counterpart to each X-ray source in the catalogue. Approximately 94\% of them are detected in the R band, while the remaining are blank elds in the optical down to R 24.5, but have a near-infrared counterpart detected by IRAC within 6 00 from the XMM\Newton centroid. The multi-band catalogue, produced using the positions of the identi ed optical counterparts, contains photometry in ten photo- metric bands, from B to the MIPS 24 m band. The spectroscopic follow-up allowed us to determine the redshift and classi cation for 237 sources ( 50\% of the sample) brighter than R = 24. The spectroscopic redshifts were complemented by reliable photometric redshifts for 68 sources. We classi ed 47\% of the sources with spectroscopic redshift as broad-line active galactic nuclei (BL AGNs) with z= 0 : 1 3 : 5, while sources without broad-lines (NOT BL AGNs) are about 46\% of the spectroscopic sample and are found up to z = 2 : 6. The remaining fraction is rep- resented by extended X-ray sources and stars. We spectroscopically identi ed 11 type 2 QSOs among the sources with F(2-10 keV)/F(R) > 8, with redshift between 0.9 and 2.6, high 2-10 keV luminosity (log L 2 10 keV 43.8 erg s 1 ) and hard X-ray colors suggesting large absorbing columns at the rest frame (log N H up to 23.6 cm 2 ). BL AGNs show on average blue optical-to-near-infrared colors, softer X-ray colors and X-ray-to-optical colors typical of optically selected AGNs. Conversely, narrow-line sources show redder optical colors, harder X-ray ux ratio and span a wider range of X-ray-to-optical colors. On average the Spectral Energy Distributions (SEDs) of high-luminosity BL AGNs resemble the power-law typical of unobscured AGNs. The SEDs of NOT BL AGNs are dominated by the galaxy emission in the optical/near-infrared, and show a rise in the mid-infrared which suggests the presence of an obscured active nucleus. We study the infrared-to-optical colors and near-infrared SEDs to infer the properties of the AGN host galaxies.

keywords : active,background,galaxies,general,surveys,x-ray