We show that external shocks cannot produce a variable gamma-ray burst, unless the burst is produc-ed by an extremely narrow jet (angular opening of ≲ 10-4) or if only a small fraction of the shell emits the radiation and the process is very inefficient. Internal shocks can produce the observed complex tem-poral structure provided that the source itself is variable. In this case, the observed temporal structure reflects the activity of the "inner engine " that drives the bursts. This sets direct constraints on it.
- Gamma rays: bursts
- Shock waves