An absence of neutrinos associated with cosmic-ray acceleration in gamma-ray bursts

Very energetic astrophysical events are required to accelerate cosmic raysto above 10(18) electronvolts. GRBs (c-ray bursts) have been proposed as possible candidate sources(1-3). In the GRB 'fireball' model, cosmic-rayacceleration should be accompanied by neutrinos produced in the decay of charged pions created in interactions between the high-energy cosmic-rayprotons and gamma-rays(4). Previous searches for such neutrinos found none, but the constraints were weak because the sensitivity was at best approximately equal to the predicted flux(5-7). Here we report an upper limit on the flux of energetic neutrinos associated with GRBs that is at least a factor of 3.7 below the predictions(4,8-10). This implies either that GRBs are not the only sources of cosmic rayswith energies exceeding 10(18) electronvoltsor that the efficiency of neutrino production is much lower than has been predicted.