An X-ray Spectral Model for Compton-Thick Toroidal Reprocessors

TL;DR

This paper introduces advanced Monte Carlo models for X-ray spectra of Compton-thick toroidal structures in AGNs, enabling more accurate spectral fitting and revealing significant geometric effects on observed reflection spectra.

Contribution

It presents new relativistic Green's function calculations for toroidal reprocessors, allowing arbitrary incident spectra and improving spectral modeling over previous disk-based models.

Findings

Reflection spectrum is ~6 times weaker in toroidal geometry compared to disk models.

Proper geometry consideration is crucial for accurate interpretation of reflection fractions.

The model self-consistently includes fluorescent line emissions.

Abstract

The central engines of both type 1 and type 2 AGNs are thought to harbor a toroidal structure that absorbs and reprocesses high-energy photons from the central X-ray source. If the reprocessor is Compton-thick, the calculation of emission-line and continuum spectra that are suitable for direct fitting to X-ray data is challenging because the reprocessed emission depends on the spectral shape of the incident continuum, which may not be directly observable. We present new Monte-Carlo calculations of Green's functions for a toroidal reprocessor that provide significant improvements over currently available models. The Green's function approach enables the construction of X-ray spectral fitting models that allow arbitrary incident spectra as part of the fitting process. The calculations are fully relativistic and have been performed for column densities that cover the Compton-thin to Compton-thick regime, for incident photon energies up to 500 keV. The reprocessed continuum and fluorescent line emission due to Fe Ka, Fe Kb, and Ni Ka are treated self-consistently, eliminating the need for ad hoc modeling that is currently common practice. We find that the spectral shape of the Compton-thick reflection spectrum in both the soft and hard X-ray bands in our toroidal geometry is different compared to that obtained from disk models. A key result of our study is that a Compton-thick toroidal structure that subtends the same solid angle at the X-ray source as a disk can produce a reflection spectrum that is ~6 times weaker than that from a disk. This highlights the widespread and erroneous interpretation of the so-called "reflection-fraction" as a solid angle, obtained from fitting disk-reflection models to Compton-thick sources without regard for proper consideration of geometry. (Abridged)