  We study the spectral properties of the unresolved cosmic X-ray background
(CXRB) in the 1.5-7.0 keV energy band with the aim of providing an
observational constraint on the statistical properties of those sources that
are too faint to be individually probed. We made use of the Swift X-ray
observation of the Chandra Deep Field South complemented by the Chandra data.
Exploiting the lowest instrument background (Swift) together with the deepest
observation ever performed (Chandra) we measured the unresolved emission at the
deepest level and with the best accuracy available today. We find that the
unresolved CXRB emission can be modeled by a single power law with a very hard
photon index Gamma=0.1+/-0.7 and a flux of 5(+/-3)E-12 cgs in the 2.0-10 keV
energy band (1 sigma error). Thanks to the low instrument background of the
Swift-XRT, we significantly improved the accuracy with respect to previous
measurements. These results point towards a novel ingredient in AGN population
synthesis models, namely a positive evolution of the Compton-thick AGN
population from local Universe to high redshift.
