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Issue A&A
Volume 476, Number 3, December IV 2007
Page(s) 1191 - 1203
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20077606



A&A 476, 1191-1203 (2007)
DOI: 10.1051/0004-6361:20077606

The XMM-Newton serendipitous survey

IV. Optical identification of the XMM-Newton medium sensitivity survey (XMS)
X. Barcons1, F. J. Carrera1, M. T. Ceballos1, M. J. Page2, J. Bussons-Gordo1, A. Corral1, J. Ebrero1, S. Mateos1, 3, J. A. Tedds3, M. G. Watson3, D. Baskill3, M. Birkinshaw4, T. Boller5, N. Borisov6, M. Bremer4, G. E. Bromage7, H. Brunner5, A. Caccianiga8, C. S. Crawford9, M. S. Cropper2, R. Della Ceca8, P. Derry3, A. C. Fabian9, P. Guillout10, Y. Hashimoto5, G. Hasinger5, B. J. M. Hassall7, G. Lamer11, N. S. Loaring2, 12, T. Maccacaro8, K. O. Mason2, R. G. McMahon9, L. Mirioni10, J. P. D. Mittaz2, C. Motch10, I. Negueruela10, 13, J. P. Osborne3, F. Panessa1, I. Pérez-Fournon14, J. P. Pye3, T. P. Roberts3, 15, S. Rosen2, 3, N. Schartel16, N. Schurch3, 15, A. Schwope11, P. Severgnini8, R. Sharp9, 17, G. C. Stewart3, G. Szokoly5, A. Ullán1, 18, M. J. Ward3, 15, R. S. Warwick3, P. J. Wheatley3, 19, N. A. Webb20, D. Worrall4, W. Yuan9, 21, and H. Ziaeepour2

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 $\times$ 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.


Key words: X-rays: general -- X-rays: galaxies -- X-rays: stars -- galaxies: active



© ESO 2007


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