Iron K Lines from Gamma‐Ray Bursts

Abstract

We present models for reprocessing of an intense flux of X-rays and gamma rays expected in the vicinity of gamma-ray burst sources. We consider the transfer and reprocessing of the energetic photons into observable features in the X-ray band, notably the K lines of iron. Our models are based on the assumption that the gas is sufficiently dense to allow the microphysical processes to be in a steady state, thus allowing efficient line emission with modest reprocessing mass and elemental abundances ranging from solar to moderately enriched. We show that the reprocessing is enhanced by down-Comptonization of photons whose energy would otherwise be too high to absorb in iron and that pair production can have an effect on enhancing the line production. Both ``distant'' reprocessors, such as supernova or wind remnants, and ``nearby'' reprocessors, such as outer stellar envelopes, can reproduce the observed line fluxes with Fe abundances 30-100 times above solar, depending on the incidence angle. The high incidence angles required arise naturally only in nearby models, which for plausible values can reach Fe line-to-continuum ratios close to the reported values.