  The discovery of a powerful and transient iron line feature in the X-ray
afterglow spectrum of GRB 970508 and GRB 970828, if confirmed, would be a major
breakthrough for understanding the nature the progenitor of gamma-ray bursts.
We show that a large mass of iron very close to the burster is necessary to
produce the emission line. This in itself strongly limits the possible
progenitor of the gamma-ray event, suggesting the former explosion of a
supernova, as predicted in the Supranova model (Vietri & Stella 1998). The line
emission process and the line intensity depend strongly on the age, density and
temperature of the remnant. The simultaneous observation of the iron line and
of a power-law optical afterglow lasted for one year strongly suggest that the
burst emission is isotropic. Recent observations of GRB 990123 are also
discussed.
