  The duration of the prompt emission of long gamma-ray bursts is generally
considered to be fairly similar to the duration of the activity of the engine
in the center of the progenitor star. Here, we investigate the relation between
the duration of the engine activity and that of the observed light curve, using
inputs from both numerical simulations and observations. We find that the
observed burst duration is a good proxy for the engine duration after the time
necessary for the jet to break out the star's surface is subtracted. However,
the observed duration is a function of the viewing angle and can be
significantly shorter than the duration of the engine activity. We also show
that the observed, redshift-corrected burst duration evolves only moderately
with redshift for both observations and synthetic light curves. We conclude
that the broad distribution of the observed duration of long BATSE gamma-ray
bursts is mostly accounted for by an engine lasting ~20 s, the dispersion being
due to viewing and redshift effects. Our results do not rule out the existence
of engines with very long duration. However, we find that they are constrained
to be a small minority of the BATSE detected bursts.
