The Beaming Factor and Other Open Issues in GRB Jets

The realization that the relativistic outflow in GRBs is in the form of jets has direct implication to their energy budget and their rates. This has, in turn, further implications on the nature of the inner engine. For example prior to the understanding that GRBs are beamed, events such as GRB 990123 with an isotropic equivalent energy of more than 10 erg were hard to explain. With beaming the energy output of this even is a“mere” 10 erg and is compatible with a simple stellar mass progenitor. Evidence of jetted GRBs arises from observations [1, 2] of the predicted achromatic breaks in the afterglow light curves [3, 4]. Further support is given by the comparison of long term radio calorimetry with the energy of the prompt emission [5]. The time of the jet break provides an estimate of the jet angle [4]: