The XMM-Newton survey of the ELAIS-S1 field^*

II. Optical identifications and multiwavelength catalogue of X-ray sources

¹ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy e-mail: chiara.feruglio@cea.fr
² CEA, Irfu, Service d' Astrophysique, Centre de Saclay, 91191 Gif-sur-Yvette, France
³ Dipartimento di Fisica, Università Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
⁴ ASI Science Data Center, via G. Galilei, 00044 Frascati, Italy
⁵ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁶ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁷ Dip. di Astronomia, Universitá di Padova, Vicolo dell' Osservatorio 3, 35122 Padova, Italy
⁸ Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA
⁹ INAF – Osservatorio Astronomico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹⁰ Dipartimento di Astronomia, Universitá di Bologna, via Ranzani 1, 40127 Bologna, Italy

Received: 13 February 2008
Accepted: 19 June 2008

Abstract

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 deg² 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 μm 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 , 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 μm band. The spectroscopic follow-up allowed us to determine the redshift and classification for 237 sources (~ of the sample) brighter than . 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 , while sources without broad-lines (NOT BL AGNs) are about 46%  of the spectroscopic sample and are found up to . The remaining fraction is represented by extended X-ray sources and stars. We spectroscopically identified 11 type 2 QSOs among the sources with  > 8, with redshift between 0.9 and 2.6, high 2-10 keV luminosity (log  ≥ 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.