  According the Collapsar model long gamma-ray bursts (LGRBs) involve
relativistic jets that puncture the envelope of a collapsing star, and produced
the \gamma-rays after they break out. This model provides a theoretical
framework for the well known association between LGRBs and massive stars.
However although this association is supported by a wealth of observations, to
this date there is no direct observational evidence for the emergence of the
jet from the star. In other words there is no direct evidence for the Collapsar
model. Here we show that a distinct signature of the Collapsar model is the
appearance of a plateau in the duration distribution of the prompt GRB emission
at times much shorter than the typical breakout time of the jet. This plateau
is evident in the data of all major GRB satellites, and provides a direct
evidence supporting the Collapsar model. It also enables us to place limits on
the sizes and masses of LGRB progenitors; suggests the existence of a large
population of choked (failed) GRBs; and indicates that the 2 s duration
commonly used to separate Collapsars and non-Collapsars holds for BATSE and
possibly Fermi GBM GRBs, but it is inconsistent with the duration distributions
of Swift GRBs.

High Energy Astrophysical Phenomena