On the equivalent width of the Fe K$\alpha$ line produced by a dusty absorber in active galactic nuclei

TL;DR

This study investigates how dust in the obscuring material around active galactic nuclei enhances the Fe K$ extalpha$ line's equivalent width in X-ray spectra, affecting interpretations of AGN properties.

Contribution

It introduces the first analysis of dust effects on X-ray reflection spectra in AGNs using photo-ionization models, highlighting significant EW enhancement due to dust.

Findings

Dust can increase Fe K$ extalpha$ EW by up to a factor of 8.

Dust reduces reflected continuum intensity through anisotropic scattering.

Implication that CT column densities may be overestimated if dust effects are ignored.

Abstract

Obscured AGNs provide an opportunity to study the material surrounding the central engine. Geometric and physical constraints on the absorber can be deduced from the reprocessed AGN emission. In particular, the obscuring gas may reprocess the nuclear X-ray emission producing a narrow Fe K$\alpha$ line and a Compton reflection hump. In recent years, models of the X-ray reflection from an obscuring torus have been computed; however, although the reflecting gas may be dusty, the models do not yet take into account the effects of dust on the predicted spectrum. We study this problem by analyzing two sets of models, with and without the presence of dust, using the one dimensional photo-ionization code Cloudy. The calculations are performed for a range of column densities ($22 <{\rm log}[N_H(\rm cm^{-2})]< 24.5$ ) and hydrogen densities ( $6 <{\rm log}[n_H(\rm cm^{-3})]< 8$). The calculations show the presence of dust can enhance the Fe K$\alpha$ equivalent width (EW) in the reflected spectrum by factors up to $\approx$ 8 for Compton thick (CT) gas and a typical ISM grain size distribution. The enhancement in EW with respect to the reflection continuum is due to the reduction in the reflected continuum intensity caused by the anisotropic scattering behaviour of dust grains. This effect will be most relevant for reflection from distant, predominately neutral gas, and is a possible explanation for AGNs which show a strong Fe K$\alpha$ EW and a relatively weak reflection continuum. Our results show it is an important to take into account dust while modeling the X-ray reflection spectrum, and that inferring a CT column density from an observed Fe K$\alpha$ EW may not always be valid. Multi-dimensional models are needed to fully explore the magnitude of the effect.