The effect of neutrinos on the initial fireballs in gamma-ray bursts

Abstract

We investigate the fate of very compact, sudden energy depositions that may lie at the origin of gamma-ray bursts (GRBs). Following on from the work of Cavallo & Rees, we take account of the much higher energies now believed to be involved. The main effect of this is that thermal neutrinos are present and energetically important. We show that these may provide sufficient cooling to tap most of the explosion energy. However, at the extreme energies usually invoked for GRBs, the neutrino opacity suffices to prevent dramatic losses, provided that the heating process is sufficiently fast. In a generic case, a few tens of per cent of the initial fireball energy will escape as an isotropic millisecond burst of thermal neutrinos with a temperature of about 60MeV, which is detectable for nearby GRBs and hypernovae. For parameters we find most likely for GRB fireballs, the dominant processes are purely leptonic and thus the baryon loading of the fireball does not affect our conclusions. © 2005 RAS.