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
[363].
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.