Phase-resolved Faraday rotation in pulsars

TL;DR

This study investigates phase-resolved Faraday rotation in pulsars, revealing that interstellar scattering largely explains RM variations across pulse phases, which could impact the accuracy of previous pulsar RM measurements.

Contribution

First convincing evidence that interstellar scattering causes RM variations across pulsar pulses, clarifying the origin of phase-resolved Faraday rotation.

Findings

RM variations are due to interstellar scattering

RM variations follow the wavelength-squared law

Previous pulsar RMs may be significantly in error

Abstract

We have detected significant Rotation Measure variations for 9 bright pulsars, as a function of pulse longitude. An additional sample of 10 pulsars showed a rather constant RM with phase, yet a small degree of RM fluctuation is visible in at least 3 of those cases. In all cases, we have found that the rotation of the polarization position angle across our 1.4 GHz observing band is consistent with the wavelength-squared law of interstellar Faraday Rotation. We provide for the first time convincing evidence that RM variations across the pulse are largely due to interstellar scattering, although we cannot exclude that magnetospheric Faraday Rotation may still have a minor contribution; alternative explanations of this phenomenon, like erroneous de-dispersion and the presence of non-orthogonal polarization modes, are excluded. If the observed, phase-resolved RM variations are common amongst pulsars, then many of the previously measured pulsar RMs may be in error by as much as a few tens of rad m-2.