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

Abstract

Gamma rays with energies greater than 7 TeV from the Crab pulsar/nebula have\nbeen observed at large zenith angles, using the Imaging Atmospheric Technique\nfrom Woomera, South Australia. CANGAROO data taken in 1992, 1993 and 1995\nindicate that the energy spectrum extends up to at least 50 TeV, without a\nchange of the index of the power law spectrum. The observed differential\nspectrum is \\noindent $(2.01\\pm 0.36)\\times 10^{-13}(E/{7 TeV})^{-2.53 \\pm\n0.18} TeV^{-1}cm^{-2}s^{-1}$ between 7 TeV and 50 TeV. There is no apparent\ncut-off. The spectrum for photon energies above $\\sim$10 TeV allows the maximum\nparticle acceleration energy to be inferred, and implies that this unpulsed\nemission does not originate near the light cylinder of the pulsar, but in the\nnebula where the magnetic field is not strong enough to allow pair creation\nfrom the TeV photons. The hard gamma-ray energy spectrum above 10 TeV also\nprovides information about the varying role of seed photons for the inverse\nCompton process at these high energies, as well as a possible contribution of\n$\\pi ^{\\circ}$-gamma rays from proton collisions.\n