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

X. Barcons*, F. J. Carrera, M. T. Ceballos, M. J. Page, J. Bussons-Gordo, A. Corral, J. Ebrero, S. Mateos, J. A. Tedds, M. G. Watson, D. Baskill, M. Birkinshaw, T. Boller, N. Borisov, M. Bremer, G. E. Bromage, H. Brunner, A. Caccianiga, C. S. Crawford, M. S. CropperR. Della Ceca, P. Derry, A. C. Fabian, P. Guillout, Y. Hashimoto, G. Hasinger, B. J. M. Hassall, G. Lamer, N. S. Loaring, T. Maccacaro, K. O. Mason, R. G. McMahon, L. Mirioni, J. P. D. Mittaz, C. Motch, I. Negueruela, J. P. Osborne, F. Panessa, I. Perez-Fournon, J. P. Pye, T. P. Roberts, S. Rosen, N. Schartel, N. Schurch, A. Schwope, P. Severgnini, R. Sharp, G. C. Stewart, G. Szokoly, A. Ullan, M. J. Ward, R. S. Warwick, P. J. Wheatley, N. A. Webb, D. Worrall, W. Yuan, H. Ziaeepour

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

40 Citations (Scopus)

Abstract

Aims. X-ray sources at intermediate fluxes (a few x 10(-14) erg cm(-2) s(-1)) with a sky density of similar to 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 (similar to 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.

Original languageEnglish
Pages (from-to)1191-U35
Number of pages30
JournalAstronomy and Astrophysics
Volume476
Issue number3
DOIs
Publication statusPublished - Dec 2007

Keywords

  • X-rays : general
  • X-rays : galaxies
  • X-rays : stars
  • galaxies : active
  • ACTIVE GALACTIC NUCLEI
  • CHANDRA MULTIWAVELENGTH PROJECT
  • X-RAY SOURCES
  • DEEP ROSAT SURVEY
  • LOG S-RELATION
  • HELLAS2XMM SURVEY
  • SOURCE CATALOG
  • LOCKMAN HOLE
  • FOLLOW-UP
  • FIELD

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