Compton Shoulder Diagnostics in Active Galactic Nuclei for Probing the Metalicity of the Obscuring Compton-Thick Tori

Abstract

We analyzed the spectral shape of the Compton shoulder around the neutral Fe-K\(_\alpha\) line of the Compton-thick type II Seyfert nucleus of the Circinus galaxy. The characteristics of this Compton shoulder with respect to the reflected continuum and Fe-K\(_\alpha\) line core intensity are a powerful diagnostics tool for analyzing the structure of the molecular tori, which obscure the central engine. We applied our Monte-Carlo-based X-ray reflection spectral model to the Chandra High Energy Transmission Grating data and successfully constrained the various spectral parameters %the hydrogen column density, the inclination angle, and %the metal abundance, independently, using only the spectral data only around the Fe-K\(_\alpha\) emission line. The obtained column density and inclination angle are consistent with the previous observations and the Compton-thick type II Seyfert picture. In addition, we determined the metal abundance of the molecular torus for the case of the smooth and clumpy torus to be 1.75\(^{+0.19}_{-0.17}\) and 1.74\(\pm\)0.16 solar abundance, respectively. Such slightly over-solar abundance can be useful information for discussing the star formation rate in the molecular tori of active galactic nuclei.