Context :

Aims : 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 .

Methods : 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 .

Results : We find that the unresolved CXRB emission can be modeled by a single power law with a very hard photon index \Gamma =0.1 \pm 0.7 and a flux of 5 ^ { +3.2 } _ { -2.6 } \times 10 ^ { -12 } erg s ^ { -1 } cm ^ { -2 } deg ^ { -2 } 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 .

Conclusions : 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 .