M. Ajello, D. M. Alexander, J. Greiner, G. M. Madejski, N. Gehrels, D. Burlon
Surveys above 10 keV represent one of the the best resources to provide an
unbiased census of the population of Active Galactic Nuclei (AGN). We present
the results of 60 months of observation of the hard X-ray sky with Swift/BAT.
In this timeframe, BAT detected (in the 15--55 keV band) 720 sources in an
all-sky survey of which 428 are associated with AGN, most of which are nearby.
Our sample has negligible incompleteness and statistics a factor of \sim2
larger over similarly complete sets of AGN. Our sample contains (at least) 15
bona-fide Compton-thick AGN and 3 likely candidates. Compton-thick AGN
represent a ~5% of AGN samples detected above 15 keV. We use the BAT dataset to
refine the determination of the LogN--LogS of AGN which is extremely important,
now that NuSTAR prepares for launch, towards assessing the AGN contribution to
the cosmic X-ray background. We show that the LogN--LogS of AGN selected above
10 keV is now established to a ~10% precision. We derive the luminosity
function of Compton-thick AGN and measure a space density of
7.9$^{+4.1}_{-2.9}\times10^{-5}$\,Mpc$^{-3}$ for objects with a de-absorbed
luminosity larger than 2$\times10^{42}$\,erg s$^{-1}$. As the BAT AGN are all
mostly local, they allow us to investigate the spatial distribution of AGN in
the nearby Universe regardless of absorption. We find concentrations of AGN
that coincide spatially with the largest congregations of matter in the local
(<85 Mpc) Universe. There is some evidence that the fraction of Seyfert 2
objects is larger than average in the direction of these dense regions.
View original:
http://arxiv.org/abs/1202.3137
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