On the efficiency of production of the Fe K α emission line in neutral matter

The absolute luminosity of the Fe Kα emission line from matter illuminated by X-rays in astrophysical sources is nontrivial to calculate except when the line-emitting medium is optically-thin to absorption and scattering. We characterize the Fe Kα line flux using a dimensionless efficiency, defined as the fraction of continuum photons above the Fe K shell absorption edge threshold energy that appear in the line. The optically-thin approximation begins to break down even for column densities as small as 2×10 cm. We show how to obtain reliable estimates of the Fe Kα line efficiency in the case of cold, neutral matter, even for the Compton-thick regime. We find that, regardless of geometry and covering factor, the largest Fe Kα line efficiency is attained well before the medium becomes Compton-thick. For cosmic elemental abundances it is difficult to achieve an efficiency higher than a few percent under the most favorable conditions and lines of sight. For a given geometry, Compton-thick lines-of-sight may have Fe Kα line efficiencies that are orders of magnitude less than the maximum possible for that geometry. Configurations that allow unobscured views of a Comptonthick reflecting surface are capable of yielding the highest efficiencies. Our results can be used to estimate the predicted flux of the narrow Fe Kα line at ∼ 6.4 keV from absorption models in AGN. In particular we show that contrary to a recent claim in the literature, absorption-dominated models for the relativistic Fe Kα emission line in MCG −6-30-15 do not over-predict the narrow Fe Kα line for any column density or covering factor.