A. Nepomuk Otte, for the VERITAS Collaboration
We present the 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/1111.6610
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