The shape of pulsar radio beams

Abstract

All available data on the mean pulse profiles of pulsars and their polarization, including a large body of previously unpublished material, have been analysed in order to determine the two-dimensional form of the radio beams emitted by pulsars. The differing properties of core and conal emission discussed by Rankin are confirmed but it is suggested that the observations are best described by a gradual change in emission characteristics from the core or axial region to the outer edge of the emission beam rather than two distinct emission processes. The spectral index difference between the core and conal regions is shown to be a function of pulsar period and is greatest for short-period pulsars. It is shown that the emission within the beam boundary is often patchy and that the distribution of component locations within the beam is essentially random. The polarization properties of pulsars with interpulses are analysed and it is shown that these pulsars fall into two groups, those where the interpulse is from the opposite pole and those where all the emission is from one pole. It is proposed that only the former group be known as interpulses. The latter group forms part of a larger set having wide profiles which are believed to originate from pulsars in which the magnetic axis is nearly aligned with the rotation axis. Evidence relating to the shape of the beam boundary is considered and it is concluded that the data are consistent with circular beams. Observational evidence suggests that the beam radius is large for short-period pulsars and is approximately proportional to P−1/3, where P is the pulsar period. Based on this relation for beam radius, the inclination angle of the magnetic axis is computed for pulsars where complete conal emission is observed. It is shown that for young pulsars the distribution of inclination angle is uniform whereas old pulsars have preferentially aligned fields. The time-scale for alignment is similar to the time-scale for field decay, that is, about 107 yr. We discuss the implications of the period dependence of beam radius, patchy emission beams and alignment of the magnetic axis on the pulsar beaming fraction, that is, the proportion of pulsars beaming towards us.