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We have converted the former solar electrical plant THEMIS (French Pyrenees)into an atmospheric Cherenkov detector called CELESTE, which records gamma raysabove 30 GeV (7E24 Hz). Here we present the first sub-100 GeV detection by aground based telescope of a gamma ray source, the Crab nebula, in the energyregion between satellite measurements and imaging atmospheric Cherenkovtelescopes. At our analysis threshold energy of 60 +/- 20 GeV we measure agamma ray rate of 6.1 +/- 0.8 per minute. Allowing for 30% systematicuncertainties and a 30% error on the energy scale yields an integral gamma rayflux of I(E>60 GeV) = 6.2^{+5.3}_{-2.3} E-6 photons m^-2 s^-1. The analysismethods used to obtain the gamma ray signal from the raw data are detailed. Inaddition, we determine the upper limit for pulsed emission to be <12% of theCrab flux at the 99% confidence level, in the same energy range. Our resultindicates that if the power law observed by EGRET is attenuated by a cutoff ofform e^{-E/E_0} then E_0 < 26 GeV. This is the lowest energy probed by aCherenkov detector and leaves only a narrow range unexplored beyond the energyrange studied by EGRET.Comment: 34 pages, accepted by the Astrophysical Journa

Measurement of the Crab Flux above 60 GeV with the CELESTE Cerenkov Telescope