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Relativistic jets in collapsars are studied by numerical simulations. Such jets are believed to give rise to outbursts of high-energy emission known as gamma-ray bursts. The propagation and break out of the jets are examined in multi-dimensional numerical simulations using a special relativistic hydrodynamics code. During its propagation, the jet is collimated by the passage through the stellar mantle. As it erupts, the highly relativistic jet core is surrounded by a cocoon of less energetic, but still moderately relativistic ejecta that expands and becomes visible at larger polar angles. We predict a distribution of energy and Lorentz factor with viewing angle in the jet beam and its cocoon. These imply that what is seen may vary greatly with viewing angle. In particular, we predict the existence of a large number of low energy GRBs with mild Lorentz factors that may be related to GRB 980425/SN 1998bw and to the recently recognized cosmological X-ray flashes. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Leading models for the "central engine" of long, soft gamma-ray bursts (L/biBs) are briefly reviewed with emphasis on the collapsar model. Growing evidence supports the hypothesis that GRBs are a supernova-like phenomenon occurring in star forming regions, differing from ordinary supernovae in that a large fraction of their energy is concentrated in highly relativistic jets. The possible progenitors and physics of such explosions are discussed and the important role of the interaction of the emerging relativistic jet with the collapsing star is emphasized. This interaction may be responsible for most of the time structure seen in long, soft GRBs. What we have called "GRBs" may actually be a diverse set of phenomena with a key parameter being the angle at which the burst is observed. GRB 980425/SN 1988bw and the recently discovered hard x-ray flashes may be examples of this diversity. [ABSTRACT FROM AUTHOR]

We review the current progenitors of the rapidly accreting black holes which are thought to power gamma-ray bursts. Differences in the evolutionary scenarios of these progenitors lead to differences in the observed properties of the bursts (e.g., burst duration, burst location, and afterglow emission). We pay particular attention to the differences between helium merger and collapsar gamma-ray burst progenitors (the two leading progenitors for long-duration gamma-ray bursts). © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]