High-energy emission from pulsar polar caps is expected to occur in connection with electron-positron pair cascades close to the neutron star surface in rotation-powered pulsars. The abundant new results from Fermi, however, strongly suggest that the observed emission from pulsars originates in the outer magneto-sphere and we have yet to observe any significant component of emission coming from the polar cap. The polar cap model also predicted a high-altitude emission component from electrons undergoing continuous acceleration from the polar cap to near the light cylinder in the slot gap. Emission from the slot gap will form caustics due to aberration and time delays that bunch the radiation on the trailing field lines of both magnetic poles. While the resulting light curves can account for the variety and distribution of observed Fermi light curves, the slot gap model falls short of accounting for the total luminosity implied by observations, requiring either a larger current of accelerated particles or a larger electric field in the gap. Population synthesis and phase-resolved spectral modeling can help us build improved emission models and constrain global magnetosphere structure.
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