Long, depolarising H$\alpha$-filament towards the Monogem ring

In soft X-rays, the Monogem ring is an object with a diameter of 25{\deg} located in the Galactic anti-centre. It is believed to be a faint, evolved, local supernova remnant. The ring is also visible in the far-ultraviolet, and a few optical filaments are related. It is not seen at radio wavelengths, as other large supernova remnants are. We study a narrow about 4.5{\deg} long, faint H$\alpha$-filament, G203.7+11.5,that is seen towards the centre of the Monogem ring. It causes depolarisation and excessive Faraday rotation of radio polarisation data. Polarisation observations at $\lambda$11\ cm and $\lambda$21\ cm with the Effelsberg 100-m telescope were analysed. in addition to $WMAP$ data, extragalactic rotation measures, and H$\alpha$ data. A Faraday-screen model was applied. From the analysis of the depolarisation properties of the H$\alpha$ filament, we derived a line-of-sight magnetic field, $B_{||}$, of 26$\pm5\mu$G for a distance of 300~pc and an electron density, $n_\mathrm{e}$, of 1.6~cm$^{-3}$. The absolute largest rotation measure of G203.7+11.5 is -86$\pm3$~rad~ m$^{-2}$, where the magnetic field direction has the opposite sign from the large-scale Galactic field. We estimated the average synchrotron emissivity at $\lambda$21\ cm up to 300~pc distance towards G203.7+11.5 to about 1.1~K $T_\mathrm{b}$/kpc, which is higher than typical Milky Way values. The magnetic field within G203.7+11.5 is unexpected in direction and strength. Most likely, the filament is related to the Monogem-ring shock, where interactions with ambient clouds may cause local magnetic field reversals. We confirm earlier findings of an enhanced but direction-dependent local synchrotron emissivity.