The XMM-Newton serendipitous survey. IV. Optical identification of the XMM-Newton medium sensitivity survey (XMS)
Pérez-Fournon, I.; Motch, C.; Bromage, G. E.; Ebrero, J.; Bremer, M.; Page, M. J.; Schartel, N.; Barcons, X.; Fabian, A. C.; Hasinger, G.; Guillout, P.; Warwick, R. S.; Carrera, F. J.; Watson, M. G.; Birkinshaw, M.; Hassall, B. J. M.; Panessa, F.; Roberts, T. P.; Schwope, A.; Wheatley, P. J.; Webb, N. A.; Cropper, M. S.; Rosen, S.; Negueruela, I.; Osborne, J. P.; Caccianiga, A.; Severgnini, P.; Della Ceca, R.; Maccacaro, T.; Brunner, H.; Mateos, S.; Tedds, J. A.; Mason, K. O.; Ward, M. J.; Corral, A.; Ceballos, M. T.; Yuan, W.; Sharp, R.; Crawford, C. S.; McMahon, R. G.; Lamer, G.; Pye, J. P.; Ziaeepour, H.; Stewart, G. C.; Ullán, A.; Boller, T.; Worrall, D.; Mirioni, L.; Szokoly, G.; Loaring, N. S.; Schurch, N.; Mittaz, J. P. D.; Borisov, N.; Hashimoto, Y.; Bussons-Gordo, J.; Baskill, D.; Derry, P.
Spain, United Kingdom, Germany, Russia, Italy, France, South Africa, Australia, China
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.