Antisymmetry in the Faraday Rotation Sky Caused by a Nearby Magnetized Bubble

TL;DR

This study uses high-resolution Faraday rotation mapping of diffuse Galactic radio emission to identify a local magnetic bubble at 100 pc, revealing that local structures significantly influence observed antisymmetries in the Faraday rotation sky.

Contribution

It demonstrates that local magnetic structures, specifically a nearby HI bubble, dominate the antisymmetry observed in the Faraday rotation sky, challenging previous interpretations attributing it to the Galactic halo.

Findings

The antisymmetry is mainly caused by a local HI bubble at 100 pc.

The magnetic field in the bubble is estimated to be 20-34 microGauss.

High-resolution Faraday maps reveal fine-scale structures in the Faraday rotation sky.

Abstract

Rotation measures of pulsars and extragalactic point sources have been known to reveal large-scale antisymmetries in the Faraday rotation sky with respect to the Galactic plane and halo that have been interpreted as signatures of the mean magnetic field in the Galactic halo. We describe Faraday rotation measurements of the diffuse Galactic polarized radio emission over a large region in the northern Galactic hemisphere. Through application of Rotation Measure Synthesis we achieve sensitive Faraday rotation maps with high angular resolution, capable of revealing fine-scale structures of about 1 deg in the Faraday rotation sky. Our analysis suggests that the observed antisymmetry in the Faraday rotation sky at b > 0 deg is dominated by the magnetic field around a local HI bubble at a distance of approx. 100 pc, and not by the magnetic field of the Galactic halo. We derive physical properties of the magnetic field of this shell, which we find to be 20 - 34 uG strong. It is clear that the diffuse polarized radio emission contains important information about the local magneto-ionic medium, which cannot yet be derived from Faraday rotation measures of extragalactic sources or pulsars alone.