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Abstract
Very energetic astrophysical events are required to accelerate cosmic rays to above
10(18) electronvolts. GRBs (γ-ray bursts) have been proposed as possible candidate
sources. In the GRB 'fireball' model, cosmic-ray acceleration should be accompanied
by neutrinos produced in the decay of charged pions created in interactions between
the high-energy cosmic-ray protons and γ-rays. Previous searches for such neutrinos
found none, but the constraints were weak because the sensitivity was at best approximately
equal to the predicted flux. 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.
This implies either that GRBs are not the only sources of cosmic rays with energies
exceeding 10(18) electronvolts or that the efficiency of neutrino production is much
lower than has been predicted.