Gamma rays with energies greater than 7 TeV from the Crab pulsar/nebula have
been observed at large zenith angles, using the Imaging Atmospheric Technique
from Woomera, South Australia. CANGAROO data taken in 1992, 1993 and 1995
indicate that the energy spectrum extends up to at least 50 TeV, without a
change of the index of the power law spectrum. The observed differential
spectrum is \noindent (2.01±0.36)×10−13(E/7TeV)−2.53±0.18TeV−1cm−2s−1 between 7 TeV and 50 TeV. There is no apparent
cut-off. The spectrum for photon energies above ∼10 TeV allows the maximum
particle acceleration energy to be inferred, and implies that this unpulsed
emission does not originate near the light cylinder of the pulsar, but in the
nebula where the magnetic field is not strong enough to allow pair creation
from the TeV photons. The hard gamma-ray energy spectrum above 10 TeV also
provides information about the varying role of seed photons for the inverse
Compton process at these high energies, as well as a possible contribution of
π∘-gamma rays from proton collisions.Comment: 19 pages, 4 figures, LaTeX2.09 with AASTeX 4.0 maros, to appear in
Astrophys. J. Let