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Jeremy J. Drake (Harvard-Smithsonian Center for Astrophysics), Douglas A. Swartz (MSFC)
Monte Carlo computations of photospheric fluorescent X-ray spectra are described. X-rays emitted from a hot (T \ge 106~K) corona incident on an underlying photosphere can undergo either scattering events by Compton or Thompson scattering, or photoabsorption events through the ionization of atoms or weakly ionized species. Through scattering events, photons can be reflected back in a direction towards the stellar surface where they have a finite chance of escape. Photoionization events involving X-ray photons directed toward the photosphere are predominantly inner shell interactions. Observable fluorescent lines can arise as a result of the finite escape probabilites of photons emitted in outward directions by hole transitions in these atoms photoionized in their inner shells. For a given stellar composition, fluorescent line strengths are dependent on the height of the fluorescing source above the photosphere and on the angle between the source and observer. Although the only fluorescent lines likely to be detected in stellar coronae are Fe~K\alpha and perhaps O K\alpha, these lines could provide powerful and direct spectroscopic diagnostics of coronal geometry.
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