Different sets of X-ray observations and theoretical arguments indicate that the reprocessing of high energy radiation by Compton thick matter in the circumnuclear regions of AGN is very common, if not ubiquitous. Unfortunately, even the most sensitive hard X-ray surveys carried out with the Chandra and XMM-Newton observatories turn out to be rather inefficient for searching for the typical signatures of thick matter. Further observations, though useful in several respects, will not provide the breakthrough in this direction. In fact, according to what can be considered a solid model for the XRB, only a small fraction of the Compton thick population could be detected by present X-ray surveys below 8 - 10 keV.
A mission capable of exploring the hard X-ray sky in the 10 - 70 keV band with focusing/imaging instruments able to reach fluxes of order 10-14 ergs cm-2 s-1 would provide a quantum leap forward. As an order of magnitude estimate, the three dex jump in the limiting flux would be similar to that achieved by BeppoSAX and ASCA in the 2 - 10 keV band with respect to the HEAO1-A2 survey of Piccinotti et al. (1982). In other words, such a gain in sensitivity would push the resolved fraction of the hard (10 - 70 keV) XRB from a negligible 1% to about 30 - 40%. Sensitive, high energy surveys will open up a large volume of discovery space that so far only has been touched by BeppoSAX observations. It is important to stress that the scientific output of these observations would be even more rewarding than a mere improvement in the limiting flux, as the 10 - 70 keV band fully encompasses the peak of the XRB energy density at 30 keV.
The X-ray Evolving Universe Spectrometer (XEUS) mission, currently under study to be the next ESA cornerstone mission for X-ray astronomy, will be capable of surveying the hard X-ray sky at, or even below, 10-14 ergs cm-2 s-1 to energies of 40 - 50 keV. Obviously, the scientific objectives of such a mission would be much broader than a better sampling of the Compton thick population. The mission would be devoted to probing a significant fraction of the electromagnetic radiation from accretion onto supermassive black holes.
Acknowledgments
The author wishes to thank Marcella Brusa, Roberto Gilli, Piero Ranalli, and Cristian Vignali for the extremely valuable support and informative discussions, Amy Barger for the opportunity to write this chapter and her patience, and Gianni Zamorani for a careful reading of the manuscript and useful comments. A special thanks to the HELLAS2XMM team for the extremely pleasant collaboration.