1 Introduction

The gamma-ray emission from GRBs is probably produced by internal shocks in a relativistic wind whereas the afterglow (from X-rays to radio bands) is due to the external shock, i.e. the forward shock propagating in the ISM because of its interaction with the wind (Rees & Mészáros 1994; Wijers et al. 1997). Simultaneously, a reverse shock propagates in the wind itself. We illustrate here the possible contribution of this reverse shock to a X-ray emission continuing immediately after the gamma-rays. Such emission has been observed in the first GRB detected by Beppo-SAX, GRB 960720. We use the detailed X-ray data made available by SAX for this burst (Piro et al. 1998) to make a comparison with our theoretical results.

  

Figure 1: Initial distribution of the Lorentz factor in the wind