The most obscured AGNs with Simbol-X

The Cosmic X-ray Background (CXB) synthesis models and the 'relict' Super Massive Black Hole mass function suggest that present 2-10 keV X-ray surveys are missing a large number of obscured (log N_H >23) AGNs. Most of these obscured AGN should shine above 10 keV. However, up to now only a few hundred sources are known above this energy, mainly thanks to the BeppoSAX PDS, INTEGRAL and Swift. We study the Simbol-X capabilities to find the most obscured AGNs, which are expected to be the most important contributors to the Cosmic X-ray Background, where it peaks at ∼30 keV. We show that in a single 1 Msec observation Simbol-X can detect 10-20 sources up to a 10-40 keV flux limit of 5-6×10^-15 erg cm^-2 s^-1. A simulation of a 1Msec observation of the Chandra Deep Field South shows that Simbol-X will detect in the 10-40 keV band several highly obscured AGN invisible even in the deepest Chandra exposure.