Evidence for evolving elongated pulsar beams

Abstract

The total change in the position angle of the linear polarization in 16 pulsars is analyzed (assuming the Radhakrishnan-Cooke (1969) magnetic pole model of pulsar radiation), on the basis of Backer and Rankin's (1980) observational data. The pulsar beams are on the average highly elongated, with the ratio R of the North-South to East-West dimensions being 3.0 + or - 0.4. The R estimate further increases if one allows for reasonable selection effects. The evolution of R with pulsar period P naturally accounts for the P dependence of interpulse occurrence. The evolution of the pulsar beaming fraction f with P renders more certain the recent observation of a deficit of short P pulsars in the Galaxy. Current pulsar electrodynamics and radio beam theories may have to be revised to account for the present results.