Revisit of local X-ray luminosity function of active galactic nuclei with the MAXI extragalactic survey

Abstract

We constructed a new X-ray (2-10 keV) luminosity function of Compton-thin active galactic nuclei (AGNs) in the local universe, using the first MAXI/GSC source catalog surveyed in the 4-10 keV band. The sample consists of 37 non-blazar AGNs at z = 0.002-0.2,whose identification is highly (>97%) complete. We confirmed the trend that the fraction of absorbed AGNs with N H > 1022 cm-2 rapidly decreases against the luminosity (LX), from 0.73±0.10 at LX = 10 42-43.5 erg s-1 to 0.12±0.08 at LX = 1043.5-45.5 erg s-1. The obtained luminosity function was well-fitted with a smoothly connected double power-law model whose indices are γ1 = 0.84 (fixed) and γ2 = 2.0±0.2 below and above the break luminosity, L* = 10 43.3±0.4 erg s-1, respectively. While the result of the MAXI/GSC agrees well with that of HEAO-1 at LX ≥ 10 43.5 erg s-1, it gives a larger number density at the lower luminosity range. A comparison between our luminosity function in the 2-10 keV band and that in the 14-195 keV band obtained from the Swift/BAT survey indicates that the averaged broad-band spectra in the 2-200 keV band should depend on the luminosity, approximated by Γ ∼ 1.7 for LX ≤ 1044 erg s-1, while Γ ∼ 2.0 for L X ≥ 1044 erg s-1. This trend was confirmed by the correlation between the luminosities in the 2-10 keV and 14-195 keV bands in our sample. We argue that there is no contradiction in the luminosity functions between above and below 10 keV once this effect is taken into account. © 2011. Astronomical Society of Japan.