A SEARCH FOR ULTRA-HIGH ENERGY COUNTERPARTS TO GAMMA-RAY BURSTS

A small air shower array operating over many years has been used to se arch for ultra-high energy (UHE) gamma radiation (greater than or equa l to 50 TeV) associated with gamma-ray bursts (GRBs) detected by the B ATSE instrument on the Compton Gamma-Ray Observatory (CGRO). Upper lim its for a one minute interval after each burst are presented for seven GRBs located with zenith angles theta < 20 degrees. A 4.3 sigma exces s over background was observed between 10 and 20 minutes following the onset of a GRB on 11 May 1991. The confidence level that this is due to a real effect and not a background fluctuation is 99.8%. If this ef fect is real then cosmological models are excluded for this burst beca use of absorption of UHE gamma rays by the intergalactic radiation fie lds.