Pair creation above pulsar polar caps - Geometrical structure and energetics of slot gaps

Abstract

A theory of pair creation over the polar caps of isolated, magnetized, rotating neutron stars with magnetic axis oblique to the rotation is developed, when steady, space charge limited flow occurs within a narrow polar flux tube bounded by regions of high conductivity and negligible flow. A quantitative calculation is given of the structure of the pair formation front which divides the low altitude, charge separated region of strong acceleration from the dense pair plasma flowing outward along the polar flux tube, including the structure and energetics of the slot gap formed between the pair plasma and the closed regions of the magnetosphere. Both the electron emission and the ion emission cases are incorporated in the electrodynamical theory. In short period objects with simple magnetic structure near the star, the particle acceleration power generated is found to be on the order of a few tenths of a percent of the total spin-down energy loss, more than enough to explain all the photon emission from the Crab and Vela pulsars. Most of this energy is generated in the slot gap. In dipole geometry, the energy is concentrated into two or three ''beams'' of accelerated particles with the regions of highest energymore » density filling only a small fraction of the polar flux tube. In long period objects with complex surface fields, the power generated in particle acceleration is comparable to the total spin-down energy loss rate, and is mostly created in the low altitude gap region between the star and the pair plasma. Some implications of this local particle acceleration theory for the beaming morphology of pulsars' photon emission are discussed, especially the possibility of observationally distinguishing the electron emission and the ion emission cases.« less