Andrew McCann, for the VERITAS Collaboration
We discuss the recent detection of pulsed gamma-ray emission from the Crab
Pulsar above 100 GeV with the VERITAS array of atmospheric Cherenkov
telescopes. Gamma-ray emission at theses energies is not expected in present
pulsar models. We find that the photon spectrum of pulsed emission between 100
MeV and 400 GeV can be described by a broken power law, and that it is
statistically preferred over a power law with an exponential cut-off. In the
VERITAS energy range the spectrum can be described with a simple power law with
a spectral index of -3.8 and a flux normalization at 150 GeV that is equivalent
to 1% of the Crab Nebula gamma-ray flux. The detection of pulsed emission above
100 GeV and the absence of an exponential cutoff rules out curvature radiation
as the primary gamma-ray-producing mechanism. The pulse profile exhibits the
characteristic two pulses of the Crab Pulsar at phases 0.0 and 0.4, albeit 2-3
times narrower than below 10 GeV. The narrowing can be interpreted as a tapered
particle acceleration region in the magnetosphere. Our findings require that
the emission region of the observed gamma rays be beyond 10 stellar radii from
the neutron star.
View original:
http://arxiv.org/abs/1110.4352
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