THE GAMMA-RAY BURST RATE AT HIGH PHOTON ENERGIES

Some gamma-ray burst (GRB) spectra exhibit high-energy tails with the highest photon energy detected at 18 GeV. The spectral slope of the hi gh-energy tails is sufficiently flat in vF(v) to consider the possibil ity of their detection at still higher energies. We calculate how many bursts can reasonably be expected above a given energy threshold for a cosmological distribution of bursts satisfying the observed apparent brightness distribution. The crucial point is that the gamma-ray abso rption by pair production in the intergalactic diffuse radiation field eliminates bursts from beyond the gamma-ray horizon tau(gamma gamma) similar to 1, thus drastically reducing the number of bursts at high e nergies. Our results are consistent with the nondetection of bursts by current experiments in the 100 GeV to 100 TeV energy range, For the e arthbound detector array MILAGRO, we predict a maximal GRB rate of sim ilar to 10 events per year. The Whipple Observatory can detect, under favorable conditions, similar to 1 event per year. The event rate for the HEGRA array is similar to 0.01 per year. Detection of significantl y higher rates of bursts would severely challenge cosmological burst s cenarios.