Search for gamma-ray bursts at photon energies e ≥ 10 GeV and e ≥ 80 TeV

The EAS-TOP extensive air shower array has been operating since 1992 in the search for gamma-ray bursts at primary energies E₁ ≥ 10 GeV and E₂ ≥ 80 TeV. The study is performed by searching for short transients in the cosmic-ray intensity in the single particle (E₁) and extensive air shower (E₂) counting rates at mountain altitude (2005 m above sea level). We discuss the method and the results obtained both in sky survey and in correlation with BATSE events. In both energy ranges, the observed fluctuations in the event rate obtained in the sky survey during ∼800 days of live time are compatible with the statistical fluctuations of the cosmic-ray background. A single candidate of time duration Δt ∼ 2 s and energy fluence F(10 < E < 100 GeV) = 1.7 × 10^-4/ (cos θ)^10.5 ergs cm^-2 (where θ is the unknown zenith angle) has been observed on 1992 July 15 at 13:22:26 UT in the energy range E₁ ≥ 10 GeV with significance 10.6 and 20.1 σ in two measurement channels. In the analysis made in correlation with ∼50 events detected by BATSE, no burst candidate was found in time coincidence or in the 2 hr interval around the BATSE detection time. The following ranges of upper limits F_max to the energy fluence in the time interval Δt₉₀ in which BATSE detected 90% of the counts are obtained: F_max = 2.3 × 10^-5-7.4 × 10^-3 ergs cm^-2 (10 < E < 100 GeV) F_max = 1.6 × 10^-6-3.3 × 10^-5 ergs cm^-2 (100 < E < 1000 TeV).