The Contribution of Particle Impact to the Production of Fe Kα Emission from Accreting Black Holes

The iron Kα line is perhaps the most important spectral diagnostic available in the study of accreting black holes. The line is thought to result from the reprocessing of external X-rays by the surface of the accretion disk. However, as is observed in the solar corona, illumination by energetic particles may also produce line emission. In principle, such a process may be uncorrelated with the observed X-rays and could explain some of the unexpected variability behavior of the Fe Kα line. This paper compares predictions of iron Kα flux generated by impacting electrons and protons to that from photoionization. Nonthermal power laws of electrons are considered as well as thermal distributions of electrons and virialized protons. The electrons are thought to originate in a magnetically dominated accretion disk corona, while the protons are considered in the context of a two-phase (hot/cold) accretion scenario. In each case, the Fe Kα flux from particle impact is found to be less than 1% of that produced by photoionization by a hard X-ray power law (normalized to the same energy flux as the particles). Thus, the electrons or protons must strike the disk with 10²-10⁴ times more energy flux than radiation for particle impact to be a significant producer of Fe Kα flux. This situation is difficult to reconcile with the observations of hard X-ray spectra or the proposed particle acceleration mechanisms in the accretion disk corona. Truncated accretion flows must be externally illuminated by hard X-rays in order to produce the Fe Kα line, as proton impact is very inefficient in generating line emission. In contrast to the Sun, our conclusion is that, with the possible exception for localized regions around magnetic footpoints, particle impact will not be an important contributor to the X-ray emission in accreting black holes.