Other
Scientific paper
Sep 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006head....9.0733b&link_type=abstract
American Astronomical Society, HEAD meeting #9, #7.33; Bulletin of the American Astronomical Society, Vol. 38, p.355
Other
Scientific paper
The INTEGRAL mission has been operational in the hard X-ray band since October 2002. Up to now more than 70 AGN have been detected by the imager IBIS/ISGRI.
INTEGRAL can contribute to our understanding of the AGN phenomenon by deriving high signal to noise spectra up to several hundred keV. By looking at the statistical properties of AGN seen by INTEGRAL, one can study the contribution the cosmic X-ray background and the role of heavily absorbed type II AGNs, which are commonly believed to be responsible for the peak at 30 keV in the extragalactic X-ray background.
We have compiled a complete extragalactic sample based on 25,000 square degrees to a limiting flux of 3E-11 ergs/cm**2/sec in the 20-40 keV band. The flux-number relation is best described by a power-law with a slope of 1.66 +- 0.11.
We present the first luminosity function of AGN in the 20-40 keV energy range, based on 38 extragalactic objects detected by the imager ISGRI. The luminosity function shows a smoothly connected two power-law form, with an index of gamma_1 = 0.8 below, and gamma_2 = 2.1 above the turn-over luminosity of Lx = 2.4E43 ergs/sec. The emissivity of all AGNs per unit volume is W(> 1E41 ergs/sec) = 2.8E38 ergs/sec/Mpc**3. These results are consistent with those derived in the 2 - 20 keV energy band and do not show a significant contribution by Compton-thick objects. Because the sample used in this study is truly local (z = 0.022), only limited conclusions can be drawn for the evolution of AGNs. But the objects explaining the cosmic X-ray background are likely to be either low luminosity AGN or of other type. An alternative scenario is an evolution of the source population, with an increasing fraction of absorbed sources with redshift.
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