The XMM-Newton survey of the ELAIS-S1 field. I: Number counts, angular   correlation function and X-ray spectral properties
  We have surveyed with XMM-Newton the central ~0.6 deg2 region of the ELAIS-S1 field down to flux limits of ~5.5X10-16 cgs (0.5-2 keV, S band), ~2X10-15 cgs (2-10 keV, H band), and ~4X10-15 cgs (5-10 keV, HH band). We detect a total of 478 sources, 395 and 205 of which detected in the S and H bands respectively. We identified 7 clearly extended sources and estimated their redshift through X-ray spectral fits with thermal models. In four cases the redshift is consistent with z=0.4. We have computed the angular correlation function of the sources in the S and H bands, finding best fit correlation angles theta_0=5.2+/-3.8 arcsec and theta_0=12.8+/-7.8 arcsec respectively. A rough estimate of the present-day correlation length r_0 can be obtained inverting the Limber equation and assuming an appropriate redshift distribution dN/dz. The results range between 12.8 and 9.8 h-1 Mpc in the S band and between 17.9 and 13.4 h-1 Mpc in the H band, with 30-40% statistical errors, assuming either smooth redshift distributions or redshift distributions with spikes accounting for the presence of a structure at z=0.4. The relative density of the S band sources is higher near the clusters and groups at z~0.4 and extends toward East and toward South/West. This suggests that the structure is complex, with a size comparable to the full XMM-Newton field. Conversely, the highest relative source densities of the H band sources are located in the central-west region of the field. 
S. Puccetti, F. Fiore, V. D'Elia, I. Pillitteri, C. Feruglio, A.   Grazian, M. Brusa, P. Ciliegi, A. Comastri, C. Gruppioni,...