The recent report of X-ray Fe features in the afterglow of the gamma-ray burst GRB 991216 may provide important clues for identifying the nature of its progenitor and constraining the burst mechanism. We argue that the strong line emission can be attributed to the interaction of a continuing (but decaying) postburst relativistic outflow from the central engine with the progenitor stellar envelope at distances less than a light-hour. Only a small mass of Fe is then required, which could have been readily produced by the star itself.
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Rees et al. (2000) studied this question.