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M. G. Baring, A. K. Harding (NASA Goddard Space Flight Center)
The EGRET experiment on the Compton Gamma-Ray Observatory significantly increased our understanding of gamma-ray pulsars by extending the database to around 10 identified sources. The upcoming GLAST experiment is expected to make the next dramatic advances in this field. Its improved sensitivity, extended spectral range and ability to perform independent (i.e. not radio or X-ray selected) pulsation searches are anticipated to provide detections of over 60, and possibly several hundred pulsars, depending on whether polar cap or outer gap scenarios control the emission properties. This paper considers expectations for the high energy spectral shape of gamma-ray pulsars using the polar cap model, and how such shape influences predictions of pulsar observability. Foremost among our considerations is the high energy spectral cutoff, which in the polar cap scenario generally declines with increasing field strength due to (i) the behavior of quantum attenuation processes of pair creation and photon splitting, and (ii) the reduction in the mean altitude of emission. Such trends are suggested by the current EGRET/Comptel population. The spectral cutoffs, which are coupled to pulsar period P and period derivative, as well as viewing angle, strongly influence calculations of observability, and have not been considered in past rank-orderings for EGRET searches. The dependence of the hard gamma-ray spectral slope below the cutoffs on pair cascade properties is outlined. Using these properties and the already established dependence of luminosity on pulsar P and period derivative, we generate a new rank-ordered listing of pulsars in preparation for the GLAST launch, tailored for different low energy thresholds below 100 MeV. We focus on different classes of pulsars, such as the highly-magnetized ones, for which the choice of the threshold in the final GLAST design critically impacts their observability. Our spectral study also provides distinctive observational tests that will enable discrimination between the polar cap and outer gap models in the GLAST era.