The XMM-Newton survey of the ELAIS-S1 field. I. Number counts, angular correlation function and X-ray spectral properties

Aims.The formation and evolution of cosmic structures can be probed by studying the evolution of the luminosity function of the Active Galactic Nuclei (AGNs), galaxies and clusters of galaxies and of the clustering of the X-ray active Universe, compared to the IR-UV active Universe.
Methods: .To this purpose, we have surveyed with XMM-Newton the central ~0.6 deg² region of the ELAIS-S1 field down to flux limits of ~5.5 × 10^-16 erg~cm^-2~s^-1 (0.5-2 keV, soft band, S), ~2 × 10^-15 erg~cm^-2~s^-1 (2-10 keV, hard band, H), and ~4 × 10^-15 erg~cm^-2~s^-1 (5-10 keV, ultra hard band, HH). We present here the analysis of the XMM-Newton observations, the number counts in different energy bands and the clustering properties of the X-ray sources.
Results: .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, so we may have detected a large scale structure formed by groups and clusters of galaxies through their hot intra-cluster gas emission. We have computed the angular correlation function of the sources in the S and H bands finding best fit correlation angles θ_0=5.2 ± 3.8 arcsec and θ_0=12.8 ± 7.8 arcsec in the two bands respectively. The correlation angle of H band sources is therefore formally ~2.5 times that of the S band sources, although the difference is at only ~1σ confidence level. A rough estimate of the present-day correlation length r0 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 significant 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.