ALBERT

Description: Context. In soft X-rays, the Monogem ring is an object with a diameter of 25° 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. Aims. We study a narrow about 4.°5 long, faint Hα-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. Methods. Polarisation observations at λ11 cm and λ21 cm with the Effelsberg 100-m telescope were analysed in addition to WMAP data, extragalactic rotation measures, and Hα data. A Faraday-screen model was applied. Results. From the analysis of the depolarisation properties of the Hα filament, we derived a line-of-sight magnetic field, B||, of 26 ± 5 μG for a distance of 300 pc and an electron density, ne, of 1.6 cm−3. The absolute largest rotation measure of G203.7 + 11.5 is −86 ± 3 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 λ21 cm up to 300 pc distance towards G203.7 + 11.5 to about 1.1 K Tb/kpc, which is higher than typical Milky Way values. Conclusions. 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.