further the argument in favor of internal shocks in these bursts and requires more contrived and fine tuned external shocks models for variability.

Kobayashi, Piran & Sari (1997) have shown that internal shocks cannot convert all the relativistic kinetic energy to gamma-rays. The remaining kinetic energy is dissipated latter when the relativistic ejecta is slowed down by the surrounding medium. The resulting external shocks produce the afterglow. An essential feature of this internal-external shocks scenario is that the afterglow is not a direct extrapolation of the initial $\gamma$-ray emission. This model suggests (Sari, 1997) for long bursts an overlap between the GRB and initial phase of afterglow. This have been indeed observed in several cases as a build up of a softer component during the GRB and a corresponding transition in the spectrum of long GRBs to a soft x-ray dominated stage towards the end of the burst.

For short GRBs the internal-external scenario suggests that the afterglow will begin few dozen seconds after the end of a short burst that was produced by internal shocks (Sari, 1997). It also suggest that this afterglow will not be a direct extrapolation of the GRB (as the two are produced by different 