Measurement of the Crab flux above 60 GeV with the Celeste Cerenkov telescope

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 (7 x 10(24) 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(-2.3)(+5.3) x 10(-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 E-0 < 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.