Iron line afterglows: General constraints

The discovery of a powerful and transient iron line feature in the X-ray af terglow spectra of gamma-ray bursts would be a major breakthrough for under standing the nature of their progenitors, strongly suggesting the presence of a large, iron rich, mass in the vicinity of the burst event. Model-indep endent 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 t o constrain the amount of beaming or anisotropy of the burst emission.