Rotation measure synthesis at the 2 m wavelength of the FAN region: Unveiling screens and bubbles

TL;DR

This study uses rotation measure synthesis at 2m wavelength to reveal and analyze complex magnetic and ionized structures in the interstellar medium of the FAN region, identifying features like the Local Bubble wall and a potential relic Stromgren sphere.

Contribution

First application of cross-correlation in Faraday depth space to characterize polarized structures, revealing new insights into interstellar magnetic field morphology and turbulence.

Findings

Detection of magnetic field reversal along the line of sight.

Identification of structures associated with the Local Bubble wall.

Evidence of turbulence in polarized structures.

Abstract

Rotation Measure synthesis (RM synthesis) of the Westerbork Synthesis Radio Telescope (WSRT) observations at 2 m wavelength of the FAN region at l=137deg, b=+7deg shows the morphology of structures in the ionized interstellar medium. We interpret the diffuse polarized synchrotron emission in terms of coherent structures in the interstellar medium and the properties of the interstellar magnetic field. For the first time, cross-correlation is applied to identify and characterize polarized structures in Faraday depth space. Complementary information about the medium are derived from H$\alpha$ emission, properties of nearby pulsars, and optical polarized starlight measurements. Three morphological patterns are recognized, showing structures on scales from degrees down to the beam size. At low Faraday depth values, a low gradient across the imaged field is detected, almost aligned with the Galactic plane. Power spectra of polarized structures in Faraday depth space provide evidence of turbulence. A sign reversal in Faraday depth space indicates a reversal of the magnetic field component along the line of sight, from towards the observer and nearby to away from the observer at larger distances. The distance to the nearby, extended component is estimated to be lesser than 100 pc, which suggests that this structure corresponds to the Local Bubble wall. For the circular component, various physical interpretations are discussed. The most likely explanation is that the circular component seems to be the presence of a nearby (about 200 pc away) relic Stromgren sphere, associated with an old unidentified white dwarf star and expanding in a low-density environment.