The possibility that gamma-ray bursts (GRBs) were not isotropic emissions was
devised theoretically as a way to ameliorate the huge energetic budget implied
by the standard fireball model for these powerful phenomena. However, the
mechanism by which after the quasy-isotropic release of a few 1050erg
yields a collimated ejection of plasma could not be satisfactory explained
analytically. The reason being that the collimation of an outflow by its
progenitor system depends on a very complex and non-linear dynamics. That has
made necessary the use of numerical simulations in order to shed some light on
the viability of some likely progenitors of GRBs. In this contribution I will
review the most relevant features shown by these numerical simulations and how
they have been used to validate the collapsar model (for long GRBs) and the
model involving the merger of compact binaries (for short GRBs).Comment: 8 pages, 1 figure. Proceedings of the conference: "Circumstellar
Media and Late Stages of Massive Stellar Evolution". Ensenada (Mexico). To be
published by Revista Mexicana de Astronomia y Astrofisic
