X-ray Fluorescent Fe Kalpha Lines from Stellar Photospheres

TL;DR

This paper uses Monte Carlo simulations to analyze how stellar photospheric Fe Kalpha lines are generated by X-ray fluorescence, providing insights into stellar geometry, metallicity, and flare characteristics.

Contribution

The study presents detailed simulations of Fe Kalpha fluorescence from stellar photospheres, linking line strength to coronal source height, spectrum, and metallicity, and offers explanations for observed stellar X-ray lines.

Findings

Fe Kalpha lines indicate flared disk geometries or high Fe abundance in pre-main sequence stars.

X-ray fluorescence can explain the ~1400 mA line during a flare on V1486 Ori without additional ionization mechanisms.

Observed Fe Kalpha lines during large stellar flares are consistent with X-ray excitation, not electron impact.

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 Kalpha 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 ionising 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 super-solar disk Fe abundances; (2) the extreme ~1400 mA line observed from a flare on V1486 Ori can 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 ionisation contribution. There is no convincing evidence supporting the energetically challenging explanation of electron impact excitation for observed stellar Fe Kalpha lines.