  The discovery of a powerful and transient iron line feature in the X-ray
afterglow spectra of gamma-ray bursts would be a major breakthrough for
understanding the nature of their progenitors. Piro et al. (1999) and Yoshida
et al. (1999) report such a detection in the afterglow of GRB 970508 and GRB
970828, respectively. We discuss how such a strong line could be produced in
the various scenarios proposed for the event progenitor. We show that the
observed line intensity requires a large iron mass, concentrated in the
vicinity of the burst. The previous explosion of a supernova, predicted in the
Supranova scenario, is the most straightforward way to account for such a large
amount of matter. We discuss three different physical processes that could
account for the line: recombination, reflection and thermal emission. Among
these, reflection and thermal emission may explain the observed line features:
reflection should be important if the remnant is optically thick, while thermal
lines can be produced only in a thin plasma. The recombination process requires
extremely high densities to efficiently reprocess the burst photons, whereas
this process could work during the X-ray afterglow. Future key observations for
discriminating the actual radiating process are discussed.
