Fe K α emission from photoionized slabs: the impact of the iron abundance

Iron K α emission from photoionized and optically thick material is observed in a variety of astrophysical environments including X‐ray binaries, active galactic nuclei and possibly gamma‐ray bursts. This paper presents calculations showing how the equivalent width (EW) of the Fe K α line depends on the iron abundance of the illuminated gas and its ionization state – two variables subject to significant cosmic scatter. Reflection spectra from a constant density slab which is illuminated with a power‐law spectrum with photon‐index Γ are computed using the code of Ross & Fabian. When the Fe K α EW is measured from the reflection spectra alone, we find that it can reach values greater than 6 keV if the Fe abundance is about 10 times the solar value and the illuminated gas is neutral. EWs of about 1 keV are obtained when the gas is ionized. In contrast, when the EW is measured from the incident+reflected spectrum, the largest EWs are ∼800 keV and are found when the gas is ionized. When Γ is increased, the Fe K α line generally weakens, but significant emission can persist to larger ionization parameters. The iron abundance has its greatest impact on the EW when it is less than 5 times the solar value. When the abundance is further increased, the line strengthens only marginally. Therefore, we conclude that Fe K α lines with EWs much greater than 800 eV are unlikely to be produced by gas with a supersolar Fe abundance. These results should be useful in interpreting Fe K α emission whenever it arises from optically thick fluorescence.