Thermal neutrinos from hot GRB fireballs

Abstract

We consider the physics of neutrinos in a fireball, i.e. a tightly coupled plasma of photons, positrons and electrons. Such a fireball is believed to form in the first stages of a gamma-ray burst. We assume the fireball is radiation-dominated and spherically symmetric. Energy considerations limit the allowed baryon density, from which it follows that the neutrino physics is dominated by leptonic processes. We find that, for quite general initial conditions, neutrinos start out in thermodynamic equilibrium with the fireball and follow the usual hydrodynamical evolution. As the fireball cools, the plasma becomes transparent to neutrinos which subsequently decouple from the plasma. Although a sizable fraction of the total energy is carried away, the detection possibility of these neutrino bursts is limited due to the isotropic outflow and the relatively low mean energy of approximately 60 MeV.