The FeK$\alpha$ Compton shoulder in accreting Supermassive Black Holes

TL;DR

This study investigates how the shape of the FeKα Compton shoulder in X-ray spectra of active galactic nuclei depends on the physical and geometrical properties of the surrounding material, using simulations to assess its diagnostic potential.

Contribution

It introduces detailed simulations exploring the influence of medium properties and geometry on the FeKα Compton shoulder, assessing its usefulness for future X-ray observatories.

Findings

The Compton shoulder shape is sensitive to optical depth and column density.

Geometrical configurations significantly affect the FeKα line and its shoulder.

Future observatories can utilize the Compton shoulder to constrain circumnuclear material properties.

Abstract

Reprocessed X-ray radiation from active galactic nuclei (AGN) carries important information about the properties of the circumnuclear material around the black hole. The X-ray photons travel from the very center of the system and interact with that material often producing strong emission lines. The FeK$\alpha$ Compton shoulder is formed by fluorescent FeK$\alpha$ photons that perform Compton scatterings with the intercepting material and lose energy to form the distinct shoulder shape. In this work we use the ray-tracing code RefleX to explore how the physical properties of the medium, as well as its geometry, affect the shape of the Compton shoulder (CS). We start by running simulations using a simple toroidal reflector, to test the effect of the metal composition, metallicity, column density, dust presence and velocity on the FeK$\alpha$ line and its Compton shoulder. We confirm that the shape of the Compton shoulder is sensitive to the optical depth of the intercepting medium, which can be regulated by either changing the metal composition or the line of sight column density of the circumnuclear material. Next, we create a series of models, which feature different geometrical configurations of dust and gas, and explore how the Compton shoulder is affected by such configurations finding that components that can regulate the line-of-sight column density affect the FeK$\alpha$ and its CS. Finally, we test whether observatories such as the recently launched XRISM and future Athena will make the Compton shoulder a useful spectral feature of nearby AGN, by applying specific models on simulated spectra of the Circinus galaxy. The CS has the potential to be used to help constrain properties of the circumnuclear material yet with some limitations.