SEARCH FOR ULTRA-HIGH-ENERGY RADIATION FROM GAMMA-RAY BURSTS

A recent class of models suggests that some gamma-ray bursters may produce ultra-high-energy (greater than or similar 100 TeV) photons. Using data from the CYGNUS array, we have searched for evidence of emission of ultra-high-energy radiation coincident with gamma-ray bursts observed either by the third Interplanetary Network of satellites or by the BATSE instrument on the Compton Gamma-Ray Observatory. No statistically significant excess was found for six bursts whose locations were accurately determined by the Network, or for any point in the sky within 2 sigma of the BATSE location coordinates of 52 additional bursts. Flux upper limits depend greatly on the zenith angles of the bursts, but typical limits above 100 TeV are approximately 4 x 10(-10) cm-2 s-1. The flux upper limits for three of the bursts imply that the observed spectrum softens between 2 MeV and approximately 100 TeV. If the production spectrum does not soften between these energies, the bursts must have a cosmological origin.