  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.
