Study of Torus Structures in Low-luminosity Active Galactic Nuclei with Suzaku

Abstract

We report the results from a systematic study of the broad band X-ray spectra of low-luminosity active galactic nuclei (LLAGN) obtained with Suzaku and Swift/BAT. Our targets consist of NGC 1566, NGC 3718, NGC 4138, NGC 4941, and NGC 5273, all of which have the hard X-ray luminosities below 10 42 erg s -1 in the 14-195 keV band. Most of the spectra of the nucleus are well reproduced by a sum of partially or fully covered transmitted emission and its reflection from the accretion disk, reprocessed emission from the torus accompanied by a strong narrow iron-K line, and a scattered component (for type-2 AGN). The equivalent width of the narrow iron-K line enables us to constrain the covering fraction of the torus by comparing with a model prediction. We find that they generally agree with the obscured AGN fraction obtained from hard X-ray at similar luminosities. Our results support the implication that the averaged covering fraction of AGN tori is peaked at L 10 42-43 erg s -1 but decreases toward lower luminosities.

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