Study of Swift/Bat Selected Low-luminosity Active Galactic Nuclei Observed with Suzaku

We systematically analyze the broadband (0.5-200 keV) X-ray spectra of hard X-ray (>10 keV) selected local low-luminosity active galactic nuclei (LLAGNs) observed with Suzaku and Swift/BAT. The sample consists of 10 LLAGNs detected with Swift/BAT with intrinsic 14-195 keV luminosities smaller than 10⁴² erg s^-1 available in the Suzaku archive, covering a wide range of the Eddington ratio from 10^-5 to 10^-2. The overall spectra can be reproduced with an absorbed cut-off power law, often accompanied by reflection components from distant cold matter, and/or optically thin thermal emission from the host galaxy. In all of the objects, relativistic reflection components from the innermost disk are not required. Eight objects show a significant narrow iron-Kα emission line. Comparing their observed equivalent widths with the predictions from the Monte-Carlo-based torus model by Ikeda et al. (2009), we constrain the column density in the equatorial plane to be {log} {N}_{{H}}^{{eq}}\gt 22.7, or the torus half-opening angle θ _oa < 70°. We infer that the Eddington ratio (λ _Edd) is a key parameter that determines the torus structure of LLAGNs: the torus becomes large at λ _Edd ≳ 2 × 10^-4, whereas at lower accretion rates it is little developed. The luminosity correlation between the hard X-ray and mid-infrared (MIR) bands of the LLAGNs follows the same correlation as for more luminous AGNs. This implies that mechanisms other than AGN-heated dust are responsible for the MIR emission in low Eddington ratio LLAGNs.