Discovery of a filamentary synchrotron structure connected to the coherent magnetic field in the outer Galaxy

Using data from the Galactic Arecibo L-band Feed Array Continuum Transit Survey (GALFACTS), we report the discovery of two previously unidentified, very compressed, thin, and straight polarized filaments approximately centred at Galactic coordinates, $(l,b)=(182.5^\circ,-4.0^\circ)$, which we call G182.5--4.0. Using data from the Isaac Newton Telescope Galactic Plane Survey (IGAPS), we also find straight, long, and extremely thin H$\alpha$ filaments coincident with the radio emission. These filaments are positioned in projection at the edge of the Orion-Eridanus superbubble and we find evidence indicating that the filaments align with the coherent magnetic field of the outer Galaxy. We find a lower limit on the total radio flux at 1.4~GHz to be $0.7\pm0.3$~Jy with an average linearly polarized fraction of $40\substack{+30 \\ -20}\%$. We consider various scenarios that could explain the origin of these filaments, including a shell-type supernova remnant (SNR), a bow shock nebula associated with a pulsar, or relic fragments from one or more supernova explosions in the adjacent superbubble, with a hybrid scenario being most likely. This may represent an example of a new class of objects that is neither an SNR nor a bow shock. The highly compressed nature of these filaments and their alignment with Galactic plane suggests a scenario where this object formed in a magnetic field that was compressed by the expanding Orion-Eridanus superbubble, suggesting that the object is related to this superbubble and implying a distance of $\sim$400~pc.