Multi - frequency polarimetry of the Galactic radio background around 350 MHz : II . A region in Horologium around l = 137 ◦ , b = 7 ◦

We study a conspicuous ring-like structure with a radius of about 1.4 which was observed with the Westerbork Synthesis Radio Telescope (WSRT) at 5 frequencies around 350 MHz. This ring is very prominent in Stokes Q and U , and less so in polarized intensity P . No corresponding structure is visible in total intensity Stokes I , which indicates that the ring is created by Faraday rotation and depolarization processes. The polarization angle changes from the center of the ring outwards to a radius ∼ > 1.7. Thus, the structure in polarization angle is not ring-like but resembles a disk, and it is larger than the ring in P . The rotation measure RM decreases almost continuously over the disk, from RM ≈ 0 rad m at the edge, to −8 rad m in the center, while outside the ring the RM is slightly positive. This radial variation of RM yields stringent constraints on the nature of the ring-like structure, because it rules out any spherically symmetrical magnetic field configuration, such as might be expected from supernova remnants or wind-blown bubbles. We discuss several possible connections between the ring and known objects in the ISM, and conclude that the ring is a predominantly magnetic funnel-like structure. This description can explain both the field reversal from outside to inside the ring, and the increase in magnetic field, probably combined with an electron density increase, towards the center of the ring. The ring-structure in P is most likely caused by a lack of depolarization due to a very uniform RM distribution at that radius. Beyond the ring, the RM gradient increases, depolarizing the polarized emission, so that the polarized intensity decreases. In the southwestern corner of the field a pattern of narrow filaments of low polarization, aligned with Galactic longitude, is observed, indicative of beam depolarization due to abrupt changes in RM . This explanation is supported by the observed RM .