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

Context. Rotation Measure synthesis of the Westerbork Synthesis Radio Telescope (WSRT) observations at λ ∼ 2 m of the FAN region at l=137◦, b=+7◦ shows the morphology of structures in the ionized interstellar medium. Aims. We interpret the diffuse polarized synchrotron emission in terms of coherent structures in the interstellar medium and the properties of the interstellar magnetic field. Methods. We performed statistical analysis of the polarization data cube obtained through Rotation Measure synthesis. 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 Hα emission, properties of nearby pulsars, and optical polarized starlight measurements. Results. We find an overall asymmetric Faraday dispersion function in a Faraday depth range of [-13,+5] rad m−2, which is peaked around −1 rad m−2. Three morphological patterns are recognized, showing structures on scales from degrees down to the beam size. The first structure is a nearby synchrotron emission component with low Faraday depth, filling the entire field of view. The second pattern is a circular polarization structure with enhanced (negative) Faraday depth, which has the same morphology as a low-emission region within the third component. This third component is interpreted as the background in which the circular structure is embedded. 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. Conclusions. A sign reversal in Faraday depth from the nearby component to the circular component 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 large distances. The distance to the nearby, extended component is estimated as . 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 (∼ 200 pc away) relic Strömgren sphere, associated with an old unidentified white dwarf star and expanding in a low-density environment.