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

Iron Kα 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 Kα 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-indexΓ are computed using the code of Ross & Fabian. When the Fe Kα 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× solar 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 Kα 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× solar. When the abundance is further increased, the line strengthens only marginally. Therefore, we conclude that Fe Kα 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 Kα emission whenever it arises from optically thick fluorescence.