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

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 Kα emission line in the toroidal X-ray reprocessor model of Murphy & Yaqoob. The model is valid for equatorial column densities in the range 1022-1025cm-2, which comprehensively covers the Compton-thin to Compton-thick regimes. We show how the shape of the Compton shoulder and its flux relative to the core of the Fe Kα emission line depend on the torus column density and orientation, for the case of a half-opening angle of 60° and cosmic abundances. The variety of Compton shoulder profiles is greater than that for both (centrally illuminated) spherical and disc 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 disc geometries. Such a Compton shoulder is dominated by a narrow back-scattering feature peaking at ∼6.24 keV. Our results are also sensitive enough to reveal a dependence of the shape of the Compton shoulder (and its magnitude relative to the Fe Kα line core) on the spectral shape of the incident X-ray continuum. We also present results of the effect of velocity broadening on the Fe Kα line profile and find that if either the velocity width or instrument resolution is greater than a full width at half-maximum (FWHM) of ∼2000kms-1, 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 an FWHM of ∼ 7000kms-1 the Compton shoulder is not resolved at all, its only signature being a weak asymmetry in the blended line profile. This means that 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. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.