The Astrophysical Journal
The American Astronomical Society
Some gamma-ray burst (GRB) spectra exhibit high energy tails with the highest photon energy detected at 18 GeV. The spectral slope of the high-energy tails is suffieniently flat in vF, to consider the possibility of their dectection at still higher energies. We calculate how many bursts can resonably be expected above a given energy threshold for a cosmological distribution of bursts satisfying the observed apparent birghtness distribution. The crucial point is that the gamma-ray absorption by pair production in the intergalactic diffuse radiation field climinates burts from beyond the gamma-ray horizion, thus drastically reducing the number of bursts at high energies. Our results are consistent with the nondetection of bursts by current experiments in the 100 GeV to 100 TcV energy range. For the earthbound detector array MILAGRO, we predict a maximal GRB rate of ~10 events per year. The Whipple Observatory can detect, under favourable conditions, ~1 event per year. The event rate for the HEGRA array is ~0.01 per year. Dection of significantly higher rates of bursts would severely challenge cosmological bursts scenarios.
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