The Compton shoulder of the Fe Kalpha fluorescent emission line in active galactic nuclei

TL;DR

This study uses Monte Carlo simulations to analyze the properties of the Compton shoulder of the Fe Kalpha line in active galactic nuclei, revealing new features and dependencies on geometry and spectral shape.

Contribution

It provides a detailed, high-accuracy Monte Carlo model of the Compton shoulder in toroidal X-ray reprocessors, uncovering new profile types and dependencies.

Findings

A new type of Compton shoulder dominated by a narrow back-scattering feature.

The shape and magnitude of the Compton shoulder depend on the incident X-ray spectral shape.

Velocity broadening and instrument resolution significantly affect the resolvability of the Compton shoulder.

Abstract

We present new, high signal-to-noise ratio results from a Monte Carlo study of the properties of the Compton shoulder of the Fe Kalpha emission line in the toroidal X-ray reprocessor model of Murphy & Yaqoob (2009, MNRAS, 397, 1549). The model comprehensively covers the Compton-thin to Compton-thick regimes and we find that the variety of Compton shoulder profiles is greater than that for both (centrally-illuminated) spherical and disk geometries. Our Monte Carlo simulations were done with a statistical accuracy that is high enough to reveal, for the case of an edge-on, Compton-thick torus, a new type of Compton shoulder that is not present in the spherical or disk geometries. Such a Compton shoulder is dominated by a narrow back-scattering feature at ~6.24 keV. Our results also reveal a dependence of the shape of the Compton shoulder (and its magnitude relative to the Fe Kalpha line core) on the spectral shape of the incident X-ray continuum. We also show the effects of velocity broadening on the Fe Kalpha line profile and find that if either the velocity width or instrument resolution is greater than a FWHM of ~2000 km/s, the Compton shoulder begins to become blended with the line core and the characteristic features of the Compton shoulder become harder to resolve. In particular, at a FWHM of ~7000 km/s the Compton shoulder is NOT resolved at all, its only signature being a weak asymmetry in the blended line profile. Thus, CCD X-ray detectors cannot unambiguously resolve the Compton shoulder. Our results are freely available in a format that is suitable for direct spectral-fitting of the continuum and line model to real data.