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

¹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Rabiańska 8, 87-100 Toruń, Poland
e-mail: jinx@ncac.torun.pl
² Department of Physics & Astronomy, University of the Western Cape, Private Bag X17, Belville 7535, South Africa
³ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Laboratoire de Physique et Chimie de l’Environnement et de l’Espace LPC2E CNRS-Université d’Orléans, 45071 Orléans, France
⁶ Station de radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU, 18330 Nançay, France

Received: 15 March 2016
Accepted: 22 June 2016

Abstract

Context. 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.

Aims. 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.

Methods. 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.

Results. 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 Nançay 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.

Conclusions. 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.