| Issue |
A&A
Volume 641, September 2020
|
|
|---|---|---|
| Article Number | A121 | |
| Number of page(s) | 7 | |
| Section | Interstellar and circumstellar matter | |
| DOI | https://doi.org/10.1051/0004-6361/202038349 | |
| Published online | 18 September 2020 | |
Long, depolarising Hα-filament towards the Monogem ring★
1
Max-Planck-Institut für Radioastronomie,
Auf dem Hügel 69,
53121
Bonn,
Germany
e-mail: wreich@mpifr-bonn.mpg.de, preich@mpifr-bonn.mpg.de
2
Department of Astronomy, Yunnan University, and Key Laboratory of Astroparticle Physics of Yunnan Province,
Kunming
650091, PR China
e-mail: xhsun@ynu.edu.cn
Received:
5
May
2020
Accepted:
3
July
2020
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.
Key words: polarization / ISM: individual objects: Monogem Ring / ISM: magnetic fields / radio continuum: ISM
Reduced images (FITS) are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/641/A121
© W. Reich et al. 2020
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Open Access funding provided by Max Planck Society.
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