Improved constraints on the Faraday rotation towards eight fast radio bursts using dense grids of polarized radio galaxies

TL;DR

This study uses dense polarized radio galaxy grids to refine Galactic Faraday rotation constraints near eight FRBs, revealing small-scale RM structures and potential sign changes, impacting FRB host RM interpretations.

Contribution

It introduces a Bayesian interpolation method to constrain small-scale Galactic RM fluctuations using dense polarization grids around FRBs.

Findings

Detected small-scale Galactic RM structures around six FRBs.

Identified potential RM sign changes not seen in previous sparse grids.

Found that Galactic RM estimates can differ significantly from all-sky maps.

Abstract

We present 2-4 GHz observations of polarized radio galaxies towards eight fast radio bursts (FRBs), producing grids of Faraday rotation measure (RM) sources with sky densities of 9-28 polarized sources per square degree. Using a Bayesian interpolation framework, we constrain Galactic RM fluctuations below ~ 1 degree squared angular scales around the FRB positions. Despite the positions of all eight FRBs far from the Galactic plane, we constrain previously unresolved small-scale Galactic RM structures around six of the eight FRBs. In two of these fields, we find potential changes in the sign of the Galactic RM that are not captured by previous, sparsely sampled RM grid observations. Our Galactic RM estimate towards the FRBs differs between a few rad m^-2 up to ~ 40 rad m^-2 from the all-sky Galactic RM map of Hutschenreuter et al. (2022). Extrapolating our results to the known population of polarized FRB sources, we may be incorrectly interpreting the host galaxy RM for ~ 30% of the FRB source population with current RM grid observations. Measuring small-scale Galactic RM variations is crucial for identifying FRBs in low density and weakly magnetized environments, which in turn could serve as potent probes of cosmic magnetism. This framework of reconstructing continuous Galactic RM structure from RM grid observations can be readily applied to FRBs that fall in the sky coverage of upcoming large-sky radio polarization surveys of radio galaxies, such as the Very Large Array Sky Survey (VLASS) and the Polarization Sky Survey of the Universe's Magnetism (POSSUM).