2002ApJ...572L..57L

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 setup 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 timescale of the explosion leading to a gamma-ray burst.