X‐Ray–fluorescent Fe Kα Lines from Stellar Photospheres

Abstract

X-ray spectra from stellar coronae are reprocessed by the underlying photosphere through scattering and photoionization events. While reprocessed X-ray spectra reaching a distant observer are at a flux level of only a few percent of that of the corona itself, characteristic lines formed by inner shell photoionization of some abundant elements can be significantly stronger. The emergent photospheric spectra are sensitive to the distance and location of the fluorescing radiation and can provide diagnostics of coronal geometry and abundance. Here we present Monte Carlo simulations of the photospheric -->K α1,α2 doublet arising from quasi-neutral Fe irradiated by a coronal X-ray source. Fluorescent line strengths have been computed as a function of the height of the radiation source, the temperature of the ionizing X-ray spectrum, and the viewing angle. We also illustrate how the fluorescence efficiencies scale with the photospheric metallicity and the Fe abundance. Based on the results we make three comments: (1) fluorescent Fe lines seen from pre-main-sequence stars mostly suggest flared disk geometries and/or supersolar disk Fe abundances; (2) the extreme 1400 mA line observed from a flare on V1486 Ori could be explained entirely by X-ray fluorescence if the flare itself were partially eclipsed by the limb of the star; and (3) the fluorescent Fe line detected by Swift during a large flare on II Peg is consistent with X-ray excitation and does not require a collisional ionization contribution. There is no convincing evidence supporting the energetically challenging explanation of electron impact excitation for observed stellar Fe Kα lines.