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

TL;DR

This study analyzes the broadband X-ray spectra of low-luminosity active galactic nuclei (LLAGNs) observed with Suzaku and Swift/BAT, revealing how their torus structures depend on Eddington ratio and comparing their luminosity correlations with more luminous AGNs.

Contribution

It provides the first systematic broadband spectral analysis of LLAGNs, constraining torus properties and their dependence on accretion rate, and compares MIR-X-ray correlations across luminosity ranges.

Findings

Torus size correlates with Eddington ratio, larger at higher ratios.

No relativistic reflection components are required in the spectra.

MIR-X-ray luminosity correlation is consistent with more luminous AGNs.

Abstract

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 {\it Suzaku} and {\it Swift}/BAT. The sample consists of ten LLAGNs detected with {\it Swift}/BAT with intrinsic 14--195 keV luminosities smaller than $10^{42}$ erg s$^{-1}$ available in the {\it 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 objects, relativistic reflection components from the innermost disk are not required. Eight objects show a significant narrow iron-K$\alpha$ emission line. Comparing their observed equivalent widths with the predictions from the Monte-Carlo based torus model by \cite{Ike09}, we constrain the column density in the equatorial plane to be $\log N^{\rm eq}_{\rm H} > 22.7$ or the torus half opening angle $\theta_{\rm oa} < 70^\circ$. We infer that the Eddington ratio ($\lambda_{\rm Edd}$) is a key parameter that determines the torus structure of LLAGNs: the torus becomes large at $\lambda_{\rm Edd} \gtrsim 2\times10^{-4}$, 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 one as for more luminous AGNs. This implies that other mechanisms than AGN-heated dust are responsible for the MIR emission in low Eddington ratio LLAGNs.