Detection of Gamma Rays of up to 50 T[CLC]e[/CLC]V from the Crab Nebula

Gamma rays with energies greater than 7 TeV from the Crab pulsar/nebula havebeen observed at large zenith angles, using the Imaging Atmospheric Techniquefrom Woomera, South Australia. CANGAROO data taken in 1992, 1993 and 1995indicate that the energy spectrum extends up to at least 50 TeV, without achange of the index of the power law spectrum. The observed differentialspectrum is \noindent $(2.01\pm 0.36)\times 10^{-13}(E/{7 TeV})^{-2.53 \pm0.18} TeV^{-1}cm^{-2}s^{-1}$ between 7 TeV and 50 TeV. There is no apparentcut-off. The spectrum for photon energies above $\sim$10 TeV allows the maximumparticle acceleration energy to be inferred, and implies that this unpulsedemission does not originate near the light cylinder of the pulsar, but in thenebula where the magnetic field is not strong enough to allow pair creationfrom the TeV photons. The hard gamma-ray energy spectrum above 10 TeV alsoprovides information about the varying role of seed photons for the inverseCompton process at these high energies, as well as a possible contribution of$\pi ^{\circ}$-gamma rays from proton collisions.Comment: 19 pages, 4 figures, LaTeX2.09 with AASTeX 4.0 maros, to appear in Astrophys. J. Let