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The origin of Gamma-Ray Bursts is one of the most interesting puzzles inrecent astronomy. During the last decade a consensus formed that long GRBs(LGRBs) arise from the collapse of massive stars and that short GRBs (SGRBs)have a different origin, most likely neutron star mergers. A key ingredient ofthe Collapsar model that explains how the collapse of massive stars produces aGRB is the emergence of a relativistic jet that penetrates the stellarenvelope. The condition that the emerging jet penetrates the envelope posesstrong constraints on the system. Using these constraints we show that: (i) Lowluminosity GRBs (llGRBs), a sub population of GRBs with a very low luminosities(and other peculiar properties: single peaked, smooth and soft) cannot beformed by Collapsars. llGRBs must have a different origin (most likely a shockbreakout). (ii) On the other hand regular LGRBs must be formed by Collapsars.(iii) While for BATSE the dividing duration between Collapsars andnon-Collapsar is indeed at $\sim 2$ sec, the dividing duration is different forother GRBs detectors. In particular most Swift bursts longer than 0.8 sec areof a Collapsar origin. This last results requires a revision of manyconclusions concerning the origin of Swift SGRBs which were based on thecommonly used 2 sec limit.

philosophical transactions - royal society. mathematical, physical and engineering sciences/philosophical transactions - royal society. mathematical, physical and engineering sciences

The Long, the Short and the Weak - the origin of GRBs