Mildly obscured active galaxies and the cosmic X-ray background
  The diffuse cosmic X-ray background (CXB) is the sum of the emission of
discrete sources, mostly massive black-holes accreting matter in active
galactic nuclei (AGN). The CXB spectrum differs from the integration of the
spectra of individual sources, calling for a large population, undetected so
far, of strongly obscured Compton thick AGN. Such objects are predicted by
unified models, which attribute most of the AGN diversity to their inclination
on the line of sight, and play an important role for the understanding of the
growth of black holes in the early Universe. The fraction of obscured AGN at
low redshift can be derived from the observed CXB spectrum assuming AGN
spectral templates and luminosity functions. We show that high signal-to-noise
average hard X-ray spectra, derived from more than a billion seconds of
effective exposure time with the Swift/BAT instrument, imply that mildly
obscured Compton thin AGN feature a strong reflection and contribute massively
to the CXB. A population of Compton thick AGN larger than that effectively
detected is not required, as no more than 6\% of the CXB flux can be attributed
to them. The stronger reflection observed in mildly obscured AGN suggests that
the covering fraction of the gas and dust surrounding their central engines is
a key factor in shaping their appearance. These mildly obscured AGN are easier
to study at high redshift than Compton thick sources.
