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 .
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 , 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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