Variability in gamma-ray bursts: A clue

We show that external shocks cannot produce a variable gamma-ray burst, unless the burst is produced by an extremely narrow jet (angular opening of less than or similar to 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 temporal 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.