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1 Introduction


In the internal-external scenario, the GRB is produced by internal shocks while the afterglow is produced by the interaction of the flow with the ISM. The original fireball model was invoked to explain the Gamma-Ray Bursts (GRBs) phenomena. It requires extreme relativistic motion, with a Lorentz factor $\gamma \gt 100$.

The afterglow observations, which fit the theory rather well, are considered as a confirmation of the fireball model. However, the current afterglow observations, which detect radiation from several hours after the burst onwards, do not probe the initial extreme relativistic conditions. By the time of the present observations, several hours after the burst, the Lorentz factor is less than $\sim
10$, and it is independent of the initial Lorentz factor.

Afterglow observations, a few seconds after the burst, can provide the missing information concerning the initial phase and prove the internal shock scenario. Such rapid observations are possible, in principle, with future missions (Kulkarni & Harrison, private communication).



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