A hard X-ray view of non-local AGNs

The Cosmic X-ray Background (CXB) provides a record of cosmic accretion history
onto actively accreting supermassive black holes. The direct resolution of the
CXB into discrete sources can therefore provide a census of AGN populations
across cosmic time.
Population synthesis models are able to explain the CXB as mixture of AGN
populations with different luminosities and obscuration degree provided a
consistent population of heavily obscured AGN, called Compton Thick (CT),
expected to contribute significantly (20-30%) to the CXB peak at 20-30 keV, is
postulated. Unfortunately, several parameter degeneracies in the modelings at
energies > 10 keV result in an order of magnitude uncertainty in the estimation
of the fraction of CT AGN. Furthermore, the direct detection of these sources at
10 keV,
is capable of resolving a sizable part (>~ 30%) of the CXB at energies around
its peak of emissivity and of obtaining a direct and almost unobscured view of
obscured AGN up to z~3 therefore starting to address the main concerns in the
modelings of the CXB. In this regard I will report on the first spectral
characterization of these AGN populations, through broad-band (0.5-24 keV)
spectral analysis of a hard-band selected sample of AGN from the NuSTAR survey
program.