We investigate the fate of very compact, sudden energy depositions that may
lie at the origin of gamma-ray bursts. Following on from the work of Cavallo
and Rees (1978), 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 gamma-ray bursts, the neutrino opacity suffices to prevent dramatic
losses, provided that the heating process is sufficiently fast. In a generic
case, a few tens of percent of the initial fireball energy will escape as an
isotropic millisecond burst of thermal neutrinos with a temperature of about 60
MeV, which is detectable for nearby gamma-ray bursts and hypernovae. For
parameters we find most likely for gamma-ray burst fireballs, the dominant
processes are purely leptonic, and thus the baryon loading of the fireball does
not affect our conclusions.Comment: 10 pages, 4 figures. To be submitted to MNRA