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

TL;DR

This study uses the spectral shape of the Compton shoulder around the Fe-Kα line in an active galactic nucleus to diagnose the metallicity and structure of the obscuring molecular torus, providing new insights into its composition.

Contribution

The paper introduces a Monte Carlo-based spectral model to analyze the Compton shoulder, enabling independent constraints on the torus's metallicity and geometry from spectral data.

Findings

Metallicity of the torus is approximately 1.75 times solar.

Column density and inclination angle are consistent with previous observations.

The method allows for detailed analysis of the torus structure using only spectral data.

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 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.