RM jitter: Wavelength-dependent Scatter in Rotation Measure Related to Faraday Complexity

TL;DR

This paper investigates wavelength-dependent scatter in Rotation Measure (RM), called RM jitter, caused by turbulent plasma structures, and explores how it affects the interpretation of extragalactic radio source observations.

Contribution

It introduces the concept of RM jitter, analyzes its dependence on wavelength coverage and turbulence properties, and compares simulated results with observed RM scatter.

Findings

RM jitter depends on wavelength coverage and turbulence power spectrum.

Simulations can reproduce observed high RM scatter for low polarization sources.

Plasma beyond the near-source environment influences RM scatter in more polarized sources.

Abstract

The relation between Faraday Rotation Measure (RM) and differential Faraday rotation by unresolved structure of a turbulent plasma is investigated for extragalactic radio sources. The RM scatter of a sample of sources affected by turbulent Faraday screens with identical power spectra of Faraday depth structure, is referred to as RM jitter. For fixed amplitude and slope of the power spectrum, the range of possible RMs depends on the wavelength coverage of the survey. RM jitter is independent of Faraday depth resolution as it results from the true Faraday depth dispersion and effects of wavelength-dependent depolarization. RM jitter for a flux density limited sample is sensitive to the power law index gamma of the power spectrum of Faraday depth structure. Assuming depolarization by a turbulent Faraday screen for all sources, a simulated flux-density-limited sample can reproduce the high RM scatter found by Vanderwoude et al. (2024) for sources that are less than 3% polarized. RM jitter of sources that are more than 3% polarized, is found to be smaller than the observed scatter, indicating that plasma other than the near-source environment dominates the RM scatter for the more polarized sources. The significance of RM jitter for applications of the RM grid is discussed.