Revisiting the shape of pulsar beams

Characterizing the shape and evolution of pulsar radio emission beams is important for understanding the observed emission. The various attempts by earlier workers investigating beam shapes have resulted in widely different conclusions. Using a carefully selected subset of the recently published multifrequency polarimetry observations of 300 radio pulsars (Gould & Lyne, 1998), we attempt to model the shape of pulsar beams. Assuming that the beam shape is elliptical, in general, and that it may depend on the angle between the rotation and the magnetic axes, we seek a consistent model where we also solve for the dependence of the beam size on frequency. From the six-frequency data on conal triple and multiple component profiles, we show that a) the pulsar emission beams follow a nested cone structure with at least three distinct cones, although only one or more of the cones may be active in a given pulsar; b) each emission cone is illuminated in the form of an annular ring of width typically about 20% of the cone radius. Although some slight preference is evident for a model where the beam is circular for an aligned rotator & latitudinally compressed for an orthogonal rotator, the possibility that the beam shape is circular at all inclinations is found to be equally consistent with the data. While the overall size scales as P (where P is the pulsar period) as expected from the notion of dipolar open field lines, we see no evidence in support of the beam shape evolution with pulsar period.