Revisit of Local X-Ray Luminosity Function of Active Galactic Nuclei with the MAXI Extragalactic Survey

Abstract

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$$=$ 0.002–0.2, whose identification is highly ( $>\ $ 97%) complete. We confirmed the trend that the fraction of absorbed AGNs with $N_{\rm H}$$>\ $ 10 $^{22}$ cm $^{-2}$ rapidly decreases against the luminosity ( $L_{\rm X}$ ), from 0.73 $\ \pm\ $ 0.10 at $L_{\rm X}$$=$ 10 $^{42-43.5}$ erg s $^{-1}$ to 0.12 $\ \pm\ $ 0.08 at $L_{\rm X}$$=$ 10 $^{43.5-45.5}$ erg s $^{-1}$ . The obtained luminosity function was well-fitted with a smoothly connected double power-law model whose indices are $\gamma_1$$=$ 0.84 (fixed) and $\gamma_2$$=$ 2.0 $\ \pm\ $ 0.2 below and above the break luminosity, $L_{*}$$=$ 10 $^{43.3\pm0.4}$ erg s $^{-1}$ , respectively. While the result of the MAXI/GSC agrees well with that of HEAO-1 at $L_{\rm X}$$\gtrsim$ 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 $\Gamma$$\sim$ 1.7 for $L_{\rm X}$$\lesssim$ 10 $^{44}$ erg s $^{-1}$ , while $\Gamma$$\sim$ 2.0 for $L_{\rm X}$$\gtrsim$ 10 $^{44}$ 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.