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

Abstract

We have compiled a complete extragalactic sample based on ~25,000 deg² to a limiting flux of 3×10^-11 ergs cm^-2 s^-1 (~7000 deg² to a flux limit of 10^-11 ergs cm^-2 s^-1) in the 20-40 keV band with INTEGRAL. We have constructed a detailed exposure map to compensate for effects of nonuniform exposure. The flux-number relation is best described by a power law with a slope of α=1.66+/-0.11. The integration of the cumulative flux per unit area leads to f_{20-40 keV}=2.6×10^-10 ergs cm^-2 s^-1 sr^-1, which is about 1% of the known 20-40 keV X-ray background. We present the first luminosity function of AGNs in the 20-40 keV energy range, based on 38 extragalactic objects detected by the imager IBIS-ISGRI on board INTEGRAL. The luminosity function shows a smoothly connected double-power-law form with an index of γ₁=0.8 below and γ₂=2.1 above the turnover luminosity of L_*=2.4×10⁴³ ergs s^-1. The emissivity of all INTEGRAL AGNs per unit volume is W_20-40keV(>10⁴¹ ergs s^-1)=2.8×10³⁸ ergs s^-1 h³₇₀ 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 in this energy band.