1 Introduction

Piro et al. (1998) and Yoshida et al. (1998) (this volume) report the detection of an iron emission line in the X-ray afterglow spectrum of two $\gamma$-ray bursts, GRB 970508 and GRB 970828. The detection of strong iron emission lines would unambiguously point towards the presence of a few per cent of iron solar masses concentrated in a compact region in the vicinity of the burster. The simplest way to account for this is to assume the presence of a very young supernova remnant. Thus the presence of such a line in the X-ray afterglow spectrum would represent the "Rosetta Stone'' for unveiling the burst progenitor. There are three main classes of models for the origin of GRB: Neutron Star-Neutron Star (NS-NS) mergers (Eichler et al. 1989), Hypernovae (Paczynski 1998) and Supranovae (Vietri & Stella 1998).

X-ray line emission following GRB events has been recently discussed in the Hypernova scenario by Ghisellini et al. (1998) and Boettcher et al. (1998). None of these works predict, with reasonable assumptions on the burst surroundings, iron lines strong enough to be detectable during the X-ray afterglow. Moreover, the line emission should last over a time-scale of years given the size of the emitting nebula. On the contrary, the Supranova accounts in a quite natural way for the presence of a massive remnant in the close vicinity of the bursting source.

Here we discuss three possibilities for the iron line origin: i) recombination of the supernova shell iron atoms, photoionized by the burst photons; ii) thermal emission of the shell, which, once heated by the burst emission, produces an iron blended line (mainly due to FeXXV and FeXXVI); iii) fluorescence of a very dense, compact and relatively cold supernova shell observed in reflection.

We assume that $\sim\! 1-5$ iron rich solar masses are concentrated in the burst surroundings. Some general constraints about the required mass and the size of the emitting region are discussed in Ghisellini et al. (1998) (this volume), whereas a more detailed description can be found in Lazzati et al. (1999). The cosmological parameters will be set throughout this letter to H₀ = 65 km s^-1 Mpc^-1, q₀=0.5 and $\Lambda=0$.