1 Introduction

At this meeting it has been reported the possible detection of an iron line in the X-ray afterglow spectra of two bursts: GRB 970508 detected by BeppoSAX (Piro et al. 1999, see also these proceedings) and GRB 970828 by ASCA (Yoshida et al. these proceedings). The significance of these features is admittedly not extremely compelling ($\sim 99\%$ and $\sim 98\%$ for the two events, respectively), but the implications that they bear are so important to justify a study on the radiation mechanisms that would produce it (Lazzati et al. 1999, see also these proceedings). We can also put strong constraints on the size and mass of the line emitting region in a model-independent way. These limits, which are very robust, point towards the presence, within a distance of $\sim 10^{16}$ cm from the burst, of at least 10^-3 $M_\odot$ of iron. We also stress that these emission line features, observed in bursts which also have an optical afterglow, strongly suggest that the line emitting region is not spherically symmetric, but must have some degree of anisotropy. Finally, this constrains the possible anisotropy of the burst radiation, since the line emitting region samples different line of sights than our own.

The cosmological parameters will be set throughout this paper to H₀ = 65 km s^-1 Mpc^-1, q₀=0.5 and $\Lambda=0$.