The XMM-Newton serendipitous survey^*,**

IV. Optical identification of the XMM-Newton medium sensitivity survey (XMS)

¹ Instituto de Física de Cantabria (CSIC-UC), 39005 Santander, Spain e-mail: barcons@ifca.unican.es
² Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
³ X-ray and Observational Astronomy Group, Department of Physics and Astronomy, Leicester University, Leicester LE1 7RH, UK
⁴ H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK
⁵ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, 85740 Garching, Germany
⁶ Special Astrophysical Observatory, 369167 Nizhnij Arkhyz, Russia
⁷ Centre for Astrophysics, University of Central Lancashire, Preston PRI 2HE, UK
⁸ INAF - Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
⁹ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
¹⁰ Observatoire Astronomique de Strasbourg, 11 rue de l'Université, 67000 Strasbourg, France
¹¹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 144482, Potsdam, Germany
¹² South African Large Telescope, PO Box 9, Observatory, 7935, South Africa
¹³ Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, 03080 Alicante, Spain
¹⁴ Instituto de Astrofísica de Canarias, 38200 La Laguna, Spain
¹⁵ Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
¹⁶ European Space Astronomy Centre, Apartado 50727, 28080 Madrid, Spain
¹⁷ Anglo-Australian Observatory, PO Box 296, Epping, NSW 1710, Australia
¹⁸ Centro de Astrobiología (CSIC-INTA), PO Box 50727, 28080 Madrid, Spain
¹⁹ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
²⁰ Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 04, France
²¹ National Astronomical Observatories of China/Yunnan Observatory, Phoenix Hill, PO Box 110, Kunming, Yunnan, PR China

Received: 4 April 2007
Accepted: 25 September 2007

Abstract

Aims.X-ray sources at intermediate fluxes (a few 10^-14 erg cm^-2 s^-1) with a sky density of ~100 deg^-2 are responsible for a significant fraction of the cosmic X-ray background at various energies below 10 keV. The aim of this paper is to provide an unbiased and quantitative description of the X-ray source population at these fluxes and in various X-ray energy bands.

Methods.We present the XMM-Newton Medium sensitivity Survey (XMS), including a total of 318 X-ray sources found among the serendipitous content of 25 XMM-Newton target fields. The XMS comprises four largely overlapping source samples selected at soft (0.5-2 keV), intermediate (0.5-4.5 keV), hard (2-10 keV) and ultra-hard (4.5-7.5 keV) bands, the first three of them being flux-limited.

Results.We report on the optical identification of the XMS samples, complete to 85-95%. At the flux levels sampled by the XMS we find that the X-ray sky is largely dominated by Active Galactic Nuclei. The fraction of stars in soft X-ray selected samples is below 10%, and only a few per cent for hard selected samples. We find that the fraction of optically obscured objects in the AGN population stays constant at around 15-20% for soft and intermediate band selected X-ray sources, over 2 decades of flux. The fraction of obscured objects amongst the AGN population is larger (~35-45%) in the hard or ultra-hard selected samples, and constant across a similarly wide flux range. The distribution in X-ray-to-optical flux ratio is a strong function of the selection band, with a larger fraction of sources with high values in hard selected samples. Sources with X-ray-to-optical flux ratios in excess of 10 are dominated by obscured AGN, but with a significant contribution from unobscured AGN.