Polarization Properties of Energetic Pulsars at Meterwavelengths

TL;DR

This study investigates the polarization properties of energetic pulsars at meter wavelengths, revealing two main polarization behaviors and explaining them through incoherent mode mixing of X and O modes in pulsar plasma.

Contribution

It provides new insights into the polarization behavior of pulsars below 1 GHz and explains observed features via mode mixing of orthogonal polarization modes.

Findings

High polarization (>70%) with single track PPA in some pulses.

Lower polarization (<70%) with scattered PPA and orthogonal modes.

Mode mixing explains the observed polarization features.

Abstract

Polarization behaviour shows a transition in the pulsar population, where energetic sources with higher spin-down energy loss, $\dot{E} > 10^{34}$ erg~s$^{-1}$, often have fractional linear polarisation ($L/I$) close to 100\%, while below this range $L/I$ is usually lower than 50\%. The polarisation behaviour has been primarily studied at higher frequencies above 1 GHz, and in this work we explore the single pulse polarisation behaviour in pulsars with $\dot{E} > 5\times10^{33}$ erg~s$^{-1}$ at a lower frequency range of 300-750 MHz. The polarisation behaviour can be divided into two categories, the first with $L/I>$ 70\% where the polarisation position angle (PPA) follows a single track, and a second group with $L/I <$ 70\% and scattered PPA behaviour with or without orthogonal modes. However, there are some single pulses in the first category that also have lower $L/I$ and exhibit the presence of two polarisation modes along orthogonal tracks. The radio emission in pulsars arises due to coherent curvature radiation (CCR) from charge bunches, which develops due to non-linear instabilities in the pulsar plasma forming charge separated envelope solitons. The CCR excites orthogonally polarised X and O modes oriented perpendicular and parallel to the magnetic field line planes, that detach in the plasma and propagate independently. The O-mode is seven times stronger than the X-mode but gets damped in the medium. We show that incoherent mixing of the X and O modes with different levels of damping can reproduce the observed polarisation features in the energetic pulsar population.