The Hard X‐Ray 20–40 keV AGN Luminosity Function

We have compiled a complete extragalactic sample based on ~25,000 deg 2 to a limiting flux of 3 × 10 -11 ergs cm -2 s -1 (~7000 deg 2 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 γ 1 = 0.8 below and γ 2 = 2.1 above the turnover luminosity of L * = 2.4 × 10 43 ergs s -1 . The emissivity of all INTEGRAL AGNs per unit volume is W 20-40 keV (> 10 41 ergs s -1 ) = 2.8 × 10 38 ergs s -1 h 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 ( = 0.022), only limited conclusions can be drawn for the evolution of AGNs in this energy band.