Rotation measure variations for 20 millisecond pulsars

TL;DR

This study investigates rotation measure variations in 20 millisecond pulsars, primarily attributing observed changes to Earth's ionosphere, with minimal evidence of long-term interstellar magnetic field variations.

Contribution

It demonstrates that ionospheric effects dominate RM variations in pulsar observations and evaluates models to correct for these effects, providing limits on interstellar RM stability.

Findings

Ionospheric models effectively correct RM variations.

Long-term interstellar RM changes are minimal (~0.1 rad m$^{-2}$ yr$^{-1}$).

Some short-term RM fluctuations are likely due to interstellar medium variability.

Abstract

We report on variations in the mean position angle of the 20 millisecond pulsars being observed as part of the Parkes Pulsar Timing Array (PPTA) project. It is found that the observed variations are dominated by changes in the Faraday rotation occurring in the Earth's ionosphere. Two ionospheric models are used to correct for the ionospheric contribution and it is found that one based on the International Reference Ionosphere gave the best results. Little or no significant long-term variation in interstellar RM was found with limits typically about 0.1 rad m$^{-2}$ yr$^{-1}$ in absolute value. In a few cases, apparently significant RM variations over timescales of a few 100 days or more were seen. These are unlikely to be due to localised magnetised regions crossing the line of sight since the implied magnetic fields are too high. Most probably they are statistical fluctuations due to random spatial and temporal variations in the interstellar electron density and magnetic field along the line of sight.