Abstract

A major portion of Fermi's legacy is centered on its approximately ten-fold increase in the gamma-ray pulsar database. The improvement in the observational archive has centered on impressive pulse phase spectroscopy in a host of pulsars, and notably the identification of exponential maximum energy turnovers in the 1-8 GeV window in the majority of Fermi pulsars. The Crab pulsar is a remarkable exception, exhibiting a broken power-law structure at the upper end of the LAT window that extends up to 120 GeV as determined by VERITAS. This paper interprets the upper end of the Crab spectrum via an exploration of the opacity due to two-photon pair creation. It is shown that the quasi-broken power-law structure observed is consistent with the action of pair attenuation in the relativistic outflow in the Crab's outer magnetosphere. This result applies to either curvature or inverse Compton mechanisms for the production of gamma-rays. An assessment of the range of altitudes on the last open field line that permit the mean photon density to be sufficient to create the observed spectral break is offered.