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

Abstract

We have converted the former solar electrical plant THEMIS (French Pyrenees)\ninto an atmospheric Cherenkov detector called CELESTE, which records gamma rays\nabove 30 GeV (7E24 Hz). Here we present the first sub-100 GeV detection by a\nground based telescope of a gamma ray source, the Crab nebula, in the energy\nregion between satellite measurements and imaging atmospheric Cherenkov\ntelescopes. At our analysis threshold energy of 60 +/- 20 GeV we measure a\ngamma ray rate of 6.1 +/- 0.8 per minute. Allowing for 30% systematic\nuncertainties and a 30% error on the energy scale yields an integral gamma ray\nflux of I(E>60 GeV) = 6.2^{+5.3}_{-2.3} E-6 photons m^-2 s^-1. The analysis\nmethods used to obtain the gamma ray signal from the raw data are detailed. In\naddition, we determine the upper limit for pulsed emission to be <12% of the\nCrab flux at the 99% confidence level, in the same energy range. Our result\nindicates that if the power law observed by EGRET is attenuated by a cutoff of\nform e^{-E/E_0} then E_0 < 26 GeV. This is the lowest energy probed by a\nCherenkov detector and leaves only a narrow range unexplored beyond the energy\nrange studied by EGRET.\n