D. The Transition from Internal Shocks to External Shocks
The internal shocks take place at a distance Rint ~ c t 2 ~ (t / 0.3 sec) 22 1014 cm. These shocks last as long as the inner engine is active. The typical observed time scale for this activity ~ 50 sec (for long bursts) and ~ 0.5 sec (for short ones). External shocks begin at Rext ~ 1016 cm. If Rext / 2 T = / c, namely if the burst is long, the afterglows begins while internal shocks are still going on and the initial part of the afterglow overlaps the late part of the GRB . At the early time the afterglow emission (from the forward shock) peaks in the high X-rays contributing also to the observed -ray flux. One can expect, therefore, a transition within the GRB from hard (pure GRB) to softer and smoother (GRB and afterglow) signal. Some observational evidence for this transition was presented in Section IIA6.