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We present optical identifications and a multi-band catalogue of a sample of478 X-ray sources in the XMM and Chandra surveys of the central 0.6 deg^2 ofthe ELAIS-S1 field. The optical/infrared counterpart of each X-ray source wasidentified using R and IRAC 3.6 um bands. This method was complemented by theprecise positions obtained through Chandra observations. Approximately 94% ofthe counterparts are detected in the R band, while the remaining are blankfields in the optical down to R~24.5, but have a near-infrared counterpartdetected by IRAC within 6 arcsec from the XMM centroid. The multi-bandcatalogue contains photometry in ten photometric bands (B to the MIPS 24 um).We determined redshift and classification for 237 sources (~50% of the sample)brighter than R=24. We classified 47% of the sources with spectroscopicredshift as broad-line active galactic nuclei (BL AGNs) with z=0.1-3.5, whilesources without broad-lines are about 46% of the spectroscopic sample and arefound up to z=2.6. We identified 11 type 2 QSOs among the sources with X/O>8,with z=0.9-2.6, high 2-10 keV luminosity (log(L2-10keV)>=43.8 erg/s) and hardX-ray colors suggesting large absorbing columns at the rest frame (logN_H up to23.6 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 selectedAGNs. Conversely, narrow-line sources show redder optical colors, harder X-rayflux ratio and span a wider range of X-ray-to-optical colors. On average theSEDs of high-luminosity BL AGNs resemble the power-law typical of unobscuredAGNs. The SEDs of NOT BL AGNs are dominated by the galaxy emission in theoptical/near-infrared, and show a rise in the mid-infrared which suggests thepresence of an obscured active nucleus.

The XMM-Newton survey of the ELAIS-S1 field II: optical identifications and multiwavelength catalogue of X-ray sources