As we have heard at this meeting, it seems likely that GRBs originate in a very small fraction of stars that undergo a catastrophic energy release event toward the end of their evolution. Expressly, the association of some GRBs with supernovae has pointed a finger at deaths of massive stars as the cause of GRBs, or at least a subset thereof. Many of their observed properties can be understood as resulting from outflows, driven by newly formed black holes, which are then subsequently collimated into a pair of anti-parallel jets within a collapsing massive star. The broad features seem to be determined by the mass of the black hole, the amount of matter surrounding it, and the orientation of the observer with respect to its axis of angular momentum. GRB jets, if powered by the black hole itself, may therefore be one of the few observable consequences of how flows close to nuclear density behave under the influence of strong gravitational fields.
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