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11th of May 2023
A broad-band population view of pulsar polarisation
by Oswald et al.
In the classic picture of radio pulsar polarization, pulsars can be anything from strongly to weakly polarized, with polarization that is predominantly linear (typically ~20%, though up to 100% in some cases) although most pulse profiles also exhibit a small amount of circular polarization (typically ~10%). The observed angle of the linear polarization (position angle or PA) evolves smoothly across the pulse profile following an S-shaped curve, which can be described as a geometrical effect of the magnetic field according to the Rotating Vector Model. According to this model, the PA profile results purely from the geometry of the pulsar and the observer’s viewing angle, and so is independent of frequency. However, it has long been known that this does not fully account for the observational picture of pulsar polarization, since individual pulsars show a wide variety of other effects, such as depolarization at high frequencies, or a frequency-dependent transition from linear to circular polarization.

Oswald et al. have undertaken a study of the broadband polarization of 271 young radio pulsars, focusing particularly on circular polarization, using high quality observations made with the Ultra-Wideband Low receiver on Murriyang, the Parkes 64m radio telescope. As an understanding of pulsar radio polarization requires both a detailed study of individual sources and a collective understanding of population-level trends, Oswald et al. provide visualizations of phase- and frequency-resolved polarization parameters of individual sources, and highlight the importance of including the impact of circular polarization and spin-down energy in overall trends.

The figure above shows the pulse profile of PSR J1900−2600 at 1414 MHz, with total intensity (black), linear polarization (red dashes), circular polarization (blue dots) and position angle (PA, black points). The jump in the Position Angle of almost 90°, at a phase bin at which the total polarization drops to zero, is suggestive of the presence of two orthogonally polarized modes of emission in pulse profiles. These orthogonal jumps are identified as being the pulse phase at which mode dominance is exchanged, such that the stronger mode and the weaker mode contribute to the observed profile swap.

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