An observational imprint of the Collapsar model of long Gamma Ray Bursts
  The Collapsar model provides a theoretical framework for the well known association between long gamma-ray bursts (GRBs) and collapsing massive stars. A bipolar relativistic jet, launched at the core of a collapsing star, drills its way through the stellar envelope and breaks out of the surface before producing the observed gamma-rays. While a wealth of observations associate GRBs with the death of massive stars, as yet there is no direct evidence for the Collapsar model itself. 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 three major satellites. These findings provide an evidence that directly supports the Collapsar model. Additionally, it suggests the existence of a large population of choked (failed) GRBs and that the 2 s duration commonly used to separate Collapsars and non-Collapasars is inconsistent with the duration distributions of {\it Swift} and Fermi GRBs and only holds for BATSE GRBs. 
Omer Bromberg, Ehud Nakar, Tsvi Piran, Re'em Sari
astro-ph.HE