  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, strongly suggesting the presence
of a large, iron rich, mass in the vicinity of the burst event.
Model-independent limits to the size and the mass of the the iron line emitting
region are derived and discussed. We also discuss how these results can be used
to constrain the amount of beaming or anisotropy of the burst emission.
