A model for distortions of polarisation-angle curves in radio pulsars

TL;DR

This paper introduces a model explaining distortions in pulsar polarisation-angle curves caused by extended microbeams, supporting a stream-based emission geometry and challenging the assumption of negligible microbeam width.

Contribution

The study presents a novel model for PA distortions in pulsars, linking microbeam spreading to observed polarisation features and supporting fan-beam emission geometry.

Findings

Model successfully interprets dissimilar polarisation effects in specific pulsars.

Explains step-like PA changes and zigzag distortions in pulse profiles.

Supports the stream-based (fan-beam) emission geometry hypothesis.

Abstract

Some radio pulsar profiles (in particular those of millisecond pulsars contain wide emission structures which cover large intervals of pulse phase. Local distortions of an average curve of polarisation angle (PA) can be identified in such profiles, and they are often found to be associated with absorption features or narrow emission components. The features may be interpreted as a convolution of a lateral profile of an emitter with a microscopic radiation pattern of a non-negligible angular extent. We study a model which assumes that such an extended microbeam of the X-mode curvature radiation is spreading the radiation polarised at a fixed position angle within an interval of pulse phase. The model is capable of interpreting the strongly dissimilar polarisation of double notches in PSR B1821-24A (for which we present new polarisation data from the Nancay Radio Telescope) and PSR J0437-4715. It also explains a step-like change in PA observed at the bifurcated trailing component in the profile of J0437-4715. A generic form of the modelled PA distortion is a zigzag-shaped wiggle, which in the presence of the second polarisation mode (O mode) can be magnified or transformed into a W- or U-shaped deflection of a total net PA. The model's efficiency in interpreting dissimilar polarisation effects provides further credence to the stream-based (fan-beam) geometry of pulsar emission. It also suggests that the microbeam width may not always be assumed negligible in comparison with the angular scale of emissivity gradients in the emission region.