SIMBAD references

We have converted the former solar electrical plant THEMIS (French Pyrenees) into an atmospheric Cerenkov detector called CELESTE, which records gamma rays above 30 GeV (7x10²⁴ Hz). Here we present the first sub-100 GeV detection by a ground-based telescope of a gamma-ray source, the Crab Nebula, in the energy region between satellite measurements and imaging atmospheric Cerenkov telescopes. At our analysis threshold energy of 60±20 GeV we measure a gamma-ray rate of 6.1±0.8 minute^–1. Allowing for 30% systematic uncertainties and a 30% error on the energy scale yields an integral gamma-ray flux of I(E>60GeV)=6.2^+5.3_–2.3x10^–6 photons m^–2 s^–1. The analysis methods used to obtain the gamma-ray signal from the raw data are detailed. In addition, we determine the upper limit for pulsed emission to be less than 12% of the Crab flux at the 99% confidence level, in the same energy range. Our result indicates that if the power law observed by EGRET is attenuated by a cutoff of form e^–E/E0_^, then E₀<26 GeV. This is the lowest energy probed by a Cerenkov detector and leaves only a narrow range unexplored beyond the energy range studied by EGRET.