Long, depolarising Hα-filament towards the Monogem ring
Abstract
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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- September 2020
- DOI:
- 10.1051/0004-6361/202038349
- arXiv:
- arXiv:2007.08221
- Bibcode:
- 2020A&A...641A.121R
- Keywords:
-
- polarization;
- ISM: individual objects: Monogem Ring;
- ISM: magnetic fields;
- radio continuum: ISM;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 7 pages, 6 figures