The origin of long, short and low-luminosity gamma-ray bursts

The origin of Gamma-Ray Bursts is one of the most interesting puzzles in recent 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 of the Collapsar model that explains how the collapse of massive stars produces a GRB is the emergence of a relativistic jet that penetrates the stellar envelope. The condition that the emerging jet penetrates the envelope poses strong constraints on the system [1]. Using these constraints we show that: (i) Low luminosity GRBs llGRBs, a sub population of GRBs with a very low luminosities (and other peculiar properties: single peaked, smooth and soft) cannot be formed by Collapsars [2]. They have a different origin (most likely a shock breakout). (ii) On the other hand regular LGRBs must be formed by Collapsars [3]. (iii) While for BATSE the dividing duration between Collapsars and non-Collapsar is indeed at ∼ 2 sec, the dividing duration is different for other GRBs detectors [4]. In particular most Swift bursts longer than 0.7 sec are of a Collapsar origin. This last results requires a revision of many conclusions concerning the origin of Swift SGRBs which were based on the classical 2 sec limit.