UC San Diego

We study steady state winds from compact objects in the regime where the wind velocity at infinity is ultrarelativistic. This may have relevance to some models of gamma-ray bursts (GRBs). Particular attention is paid to the case in which neutrinos provide the heating. Unless the neutrino luminosity is very large, L > 1054 ergs s-1, the only allowed steady state solutions are those where energy deposition is dominated by neutrino-antineutrino annihilation at the sonic point. In this case, the matter temperature near the neutron star surface is low, less than 1 MeV for typical neutrino luminosities. This is in contrast to the case for subrelativistic winds discussed in the context of supernovae, where the matter temperature near the neutron star approximates the temperature characterizing the neutrinos. We also investigate the setting of the neutron-to-proton ratio (N/P) in these winds and find that only for large (> 10 MeV) electron-neutrino or electron-antineutrino temperatures is N/P entirely determined by neutrino capture. Otherwise, N/P retains an imprint of conditions in the neutron star.