Investigations of the Emission Geometry of the Four-Component Radio Pulsar J0631+1036

Radio pulsar J0631+1036 presents a remarkably clear example of a rare four-component profile, and on closer study almost every aspect of it is strange or difficult to understand: the symmetric spacing and form of its four components; its evolution over the available 3 octaves; and the apparently large aberration/retardation indicated by its linear polarization-angle traverse. Given the broad success of the core/double-cone geometric model in describing pulsar profiles, we attempt applying this model to J0631+1036. The model values resulting for the magnetic colatitude a and sightline impact angle beta are not implausible; however, overall the double-cone model appears to fail for several different reasons. Abandoning the standard model, we explore the possibility that two of the components represent downwardly-beamed emission observed from the far side of the pulsar. We propose a simple model consisting of a single beam along the dipole axis from each pole, with the PA swing assumed to be generated by the nearside beam. This sees the first and last components as the L-band locations of the beam emission at an altitude of 1100 km, while the interior components are generated by caustic effects at lower altitude 360 km within the same beams. The model naturally gives rise to the symmetric pattern between the components with the centroid as its fiducial point, and can explain why the profile widens rapidly at low frequencies.