Fast Radio Bursts as Probes of Magnetic Fields in the Intergalactic Medium

TL;DR

This paper proposes a method to use dispersion and Faraday rotation measures of extragalactic fast radio bursts to probe the magnetic fields in the intergalactic medium, especially in filaments, without needing the FRB redshift.

Contribution

It introduces a new technique to estimate intergalactic magnetic field strength from FRB data, bypassing the need for redshift measurements.

Findings

The method accurately reproduces the magnetic field strength in simulations.

Excluding sightlines passing through galaxy clusters improves measurement accuracy.

The approach relies on DM and RM data, independent of FRB redshift.

Abstract

We examine the proposal that the dispersion measures (DMs) and Faraday rotation measures (RMs) of extragalactic linearly-polarized fast radio bursts (FRBs) can be used to probe the intergalactic magnetic field (IGMF) in filaments of galaxies. The DM through the cosmic web is dominated by contributions from the warm-hot intergalactic medium (WHIM) in filaments and from the gas in voids. On the other hand, RM is induced mostly by the hot medium in galaxy clusters, and only a fraction of it is produced in the WHIM. We show that if one excludes FRBs whose sightlines pass through galaxy clusters, the line-of-sight strength of the IGMF in filaments, $B_{||}$, is approximately $C(\langle 1+z \rangle/f_{DM})(RM/DM)$, where $C$ is a known constant. Here, the redshift of the FRB is not required to be known; $f_{DM}$ is the fraction of total DM due to the WHIM, while $\langle 1+z \rangle$ is the redshift of interevening gas weighted by the WHIM gas density, both of which can be evaluated for a given cosmology model solely from the DM of an FRB. Using data on structure formation simulations and a model IGMF, we show that $C(\langle 1+z \rangle/f_{DM})(RM/DM)$ closely reproduces the density-weighted line-of-sight strength of the IGMF in filaments of the large-scale structure.