A direct comparison of X-ray spectral models for tori in active galactic nuclei

TL;DR

This study compares different X-ray spectral models for AGN tori, verifying their accuracy using a Geant4-based code, and finds discrepancies in some models' reflection features and Fe Kα line strengths.

Contribution

It provides a direct comparison of existing spectral models against a Geant4 simulation, highlighting their limitations and potential biases in parameter estimation.

Findings

Reproduced MY09 spectra accurately for high column densities.

Found BN11 model's reflection component inconsistent with its geometry.

Identified differences in Fe Kα line strength between models and analytical results.

Abstract

Several X-ray spectral models for tori in active galactic nuclei (AGNs) are available to constrain the properties of tori; however, the accuracy of these models has not been verified. We recently construct a code for the torus using Geant4, which can easily handle different geometries (Liu & Li 2014). Thus, we adopt the same assumptions as Murphy & Yaqoob (2009, hereafter MY09) and Brightman & Nandra (2011, hereafter BN11) and try to reproduce their spectra. As a result, we can reproduce well the reflection spectra and the strength of the Fe K$\alpha$ line of MY09, for both $\NH=10^{24}$ and $10^{25}$ cm$^{-2}$. However, we cannot produce the strong reflection component of BN11 in the low-energy band. The origin of this component is the reflection from the visible inner wall of the torus, and it should be very weak in the edge-on directions under the geometry of BN11. Therefore, the behaviour of the reflection spectra in BN11 is not consistent with their geometry. The strength of the Fe K$\alpha$ line of BN11 is also different from our results and the analytical result in the optically thin case. The limitation of the spectral model will bias the parameters from X-ray spectral fitting.