Soft X-ray Emission Lines in the Early Afterglow of Gamma-ray Bursts

We compute the luminosity of Kα emission lines produced by astrophysically abundant elements in the soft X-ray spectra of the early afterglow of gamma-ray bursts. We find that the detection of these lines can be a diagnostic for the geometrical set-up of the reprocessing material. In particular we can distinguish between a “geometry dominated” model, in which the line emission is coming from an extended region and its duration arises from light-travel-time effects and an “engine dominated” model, where the line emitting gas is in a smaller region, irradiated for a longer period. These lines therefore offer clues to the dynamics and time-scale of the explosion leading to a gamma-ray burst. Subject headings: gamma rays: bursts — line: formation — radiation mechanisms: non-thermal