Collapsars
  A variety of stellar explosions powered by black hole accretion are discussed. All involve the failure of neutrino energy deposition to launch a strong supernova explosion. A key quantity which determines the type of high energy transient produced is the ratio of the engine operation time, $\rm t_{engine}$, to the time for the explosion to break out of the stellar surface, $\rm t_{bo}$. Stars with sufficient angular momentum produce collapsars -- black holes accreting rapidly through a disk -- in their centers. Collapsars can occur in stars with a wide range of radii depending on the amount of pre-collapse mass loss. The stellar radius and jet properties determine the degree of beaming of the explosion. In some cases the stellar envelope serves to focus the explosion to narrow beaming angles. The baryon loading of various models for classical GRBs formed in massive stars is examined and the consequences are explored. For $\rm t_{engine} > t_{bo}$, highly relativistic outflow is possible and classical GRBs accompanied by supernovae can be produced. In other cases hyper-energetic, asymmetric supernovae are produced. Longer GRBs ($t \gtaprx 100 \s$) can be produced by fallback following a weak neutrino-driven supernova explosion. 
Andrew MacFadyen