# Diffuse polarized emission in the LOFAR Two-meter Sky Survey

## TL;DR

This paper presents high-resolution Faraday tomography of the Galactic diffuse polarized emission using LOFAR data, revealing new polarization features and structures in the interstellar medium with potential for large-scale Galactic magnetic field mapping.

## Contribution

The study applies Faraday tomography to LOFAR LOTSS data, producing a detailed Faraday depth cube mosaic and identifying new polarization features and gradients in the Galactic ISM.

## Key findings

- Detected diffuse polarized emission with brightness 1-6 K RMSF$^{-1}$.
- Identified new polarization features up to 15 degrees long.
- Observed linear Faraday depth gradients correlated with HI filaments.

## Abstract

Faraday tomography allows us to map diffuse polarized synchrotron emission from our Galaxy and use it to interpret the magnetic field in the interstellar medium (ISM). We have applied Faraday tomography to 60 observations from the LOFAR Two-meter Sky Survey (LOTSS) and produced a Faraday depth cube mosaic covering 568 square degrees at high Galactic latitudes, at 4.3' angular resolution and 1 rad m$^{-2}$ Faraday depth resolution, with a typical noise level of 50--100 $\mu$Jy per point spread function (PSF) per rotation measure spread function (RMSF) (40-80 mK RMSF$^{-1}$). While parts of the images are strongly affected by instrumental polarization, we observe diffuse polarized emission throughout most of the field, with typical brightness between 1 and 6 K RMSF$^{-1}$, and Faraday depths between $-7$ and +25 rad m$^{-2}$. We observed many new polarization features, some up to 15 degrees in length. These include two regions with very uniformly structured, linear gradients in the Faraday depth; we measured the steepness of these gradients as 2.6 and 13 rad m$^{-2}$ deg$^{-1}$. We also observed a relationship between one of the gradients and an HI filament in the local ISM. Other ISM tracers were also checked for correlations with our polarization data and none were found, but very little signal was seen in most tracers in this region. We conclude that the LOTSS data are very well suited for Faraday tomography, and that a full-scale survey with all the LOTSS data has the potential to reveal many new Galactic polarization features and map out diffuse Faraday depth structure across the entire northern hemisphere.