1 Introduction

Distance - scale determination of Gamma-ray bursts (GRB) has been one of the most important achievements of astrophysics in recent years. Accurate and fast localization of the prompt and afterglow emission (Piro et al. 1998a; Costa et al. 1997) by BeppoSAX (Piro et al. 1995; Boella et al. 1997) led to the identification of optical counterparts (van Paradijs et al. 1997) and ultimately to spectral measurements of a redshift (Metzger et al. 1997). While the extragalactic origin of GRB has gathered solid evidence in its support, the source of the large energy implied by their distance is still speculative.

The measurement of X-ray Fe lines emitted directly by the GRB or its afterglow could provide a direct measurement of the distance and probe into the nature of the central environment (Perna & Loeb 1998; Mészáros & Rees 1998; Boettcher et al. 1999; Ghisellini et al. 1998). Neutron star - neutron star merging should happen in a fairly clean environment, with line intensities much below the sensitivity of current experiments. In contrast, Mészáros & Rees (1998) have shown that the circumburst environment created by the stellar wind before the explosion of the hypernova could yield a line of substantial intensity. A similarly favourable situation should be expected in a variation of the hypernova scenario, - the SupraNova (Vietri & Stella 1998), where the GRB is shortly preceded by a supernova explosion with the ejection of an iron-rich massive shell. It is also conceivable that the impact of the relativistic shell that produced the original GRB on these ejecta could provide an additional energy input in the afterglow.

Motivated by these expectations, we have started a detailed analysis of afterglow spectra to look for the presence of features. The first and most promising candidate is GRB 970508. It is characterized by a large outbursting event during its afterglow phase (Piro et al. 1998b) and has the highest signal to noise ratio of the BeppoSAX GRB afterglows. Here we summarize the results obtained in this burst (reported more extensively in Piro et al. 1999) and present the first results of the analysis in other afterglows.