INTEGRAL sheds new light on the local population of active galactic nuclei
INTEGRAL has performed a number of multi-year extra-galactic observing campaigns,
including deep observations of large fields around 3C 273/Coma, the Large Magellanic
Cloud and the M81 galaxy. The obtained hard X-ray maps (left figure) have been used
to extend the statistics of low-redshift active galactic nuclei (AGN) down to the
previously poorly studied flux range between 10-12 and 10-11 erg/s/cm2, in the INTEGRAL energy band.
Such large-area hard X-ray surveys provide an accurate census of active galactic
nuclei (AGN) in the local Universe. Using the INTEGRAL all-sky survey, scientists
from the Space Research Institute (IKI, Moscow) have investigated the puzzling
declining trend of the fraction of obscured AGN with increasing luminosity. They
pointed out that scattering of X-rays from the central supermassive black hole in
the obscuring torus significantly raises and lowers, respectively, the chances of
finding unobscured and obscured AGN in the 17-60 keV energy band. After correction
for this observational bias, the obscured AGN fraction still shows a declining trend
with luminosity (top right figure). However, if one also takes into account that the
central engine’s emission is possibly collimated along the axis of the torus, the
data turn out to be consistent with the half-opening angle of the torus being constant
(~30 deg) with luminosity (bottom right figure).
