We present the optical identifications 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 field .
The most likely optical/infrared counterpart of each X-ray source was identified using the chance coincidence probability in the R and IRAC 3.6 \mu m  bands .
This method was complemented by the precise positions obtained through Chandra  observations .
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 fields in the optical down to R \sim 24.5 , but have a near-infrared counterpart detected by IRAC within 6″ from the XMM–Newton  centroid .
The multi-band catalogue , produced using the positions of the identified optical counterparts , contains photometry in ten photometric bands , from B to the MIPS 24 \mu m  band .
The spectroscopic follow-up allowed us to determine the redshift and classification for 237 sources ( \sim 50 \% of the sample ) brighter than R = 24 .
The spectroscopic redshifts were complemented by reliable photometric redshifts for 68 sources .
We classified 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 \% Â of the spectroscopic sample and are found up to z = 2.6 .
The remaining fraction is represented by extended X-ray sources and stars .
We spectroscopically identified 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 } \geq 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 flux 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 .