Data Centre for Astrophysics
Astronomy Department of the University of Geneva
ISDC Seminar

Friday, 24 March 2006 at 11:00

Volker Beckmann
NASA Goddard Space Flight Center

The Hard X-ray 20-40 keV AGN Luminosity Function

Abstract. We present the first luminosity function of active galactic nuclei (AGN) in the 20-40 keV energy range, based on 70 extragalactic objects detected by the imager IBIS/ISGRI on-board INTEGRAL. It was obtained by compiling a complete, significance limited sample based on ~25,000 square degrees to a limiting flux of 3 10-11 ergs/cm2/sec (~7,000 square degrees to a flux limit of 10-11 ergs/cm2/sec) in the 20-40 keV band with INTEGRAL. We have constructed a detailed exposure map to compensate for effects of non-uniform exposure. The flux-number relation is best described by a power-law with a slope of 1.66 ± 0.11. Integration of the cumulative flux per unit area leads to fx = 2.6 10-10 ergs/cm2/sec, which is about 1% of the known 20-40 keV X-ray background.

The AGN luminosity function shows a smoothly connected two power-law form, with an index of Γ1 = 0.9 below, and Γ2 = 2.2 above the turn-over luminosity of Lx = 4.6 1043 ergs/sec. The emissivity of all INTEGRAL AGNs per unit volume is W(> 1041 ergs/sec) = 2.8 1038 ergs/sec/Mpc3. 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 in this energy band. But the objects explaining the peak in the cosmic X-ray background are likely to be either low luminosity AGN (Lx < 1041 ergs/sec) or of other type, such as intermediate mass black holes, clusters, and star forming regions.

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