Detection of an optical filament in the Monogem Ring
Abstract
The Monogem Ring is a huge bright soft X-ray enhancement with a diameter of ~25°. This 0.3 kpc distant structure is a peculiar Galactic supernova remnant in that it is obviously visible only in X-rays, due to its expansion into a region of extremely low ambient density: hence, practically no optical emission or a neutral H I shell was expected to be detectable. Here we report on the discovery of a very faint arc-like nebula on a POSS II R film copy, at the south-eastern borders of the MR. Spectroscopy revealed this filament to have a very large [S II]λ 6716+6731/Hα ratio of up to ~1.8, indicating shock excitation, and a low density of Ne < 100 cm-3. There is no hint of [O III] emission in the spectra. On deep wide-field direct images in Hα and in [S II] the nebula appears as a ~20´ long, thin (~1´), structured filament, stretching N-S. We believe that this filament belongs to the MR and became visible due to the interaction of the expanding remnant with a mild density increase in the interstellar medium. Only one other possible optical filament of the MR has been reported in the literature, but no spectrum was provided.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- March 2006
- DOI:
- 10.1051/0004-6361:20054183
- arXiv:
- arXiv:astro-ph/0511546
- Bibcode:
- 2006A&A...448.1095W
- Keywords:
-
- ISM: supernova remnants;
- ISM: individual objects:;
- Monogem Ring;
- Astrophysics
- E-Print:
- 6 pages, 4 figures, of which 2 in jpeg format and degraded because of size constraints. Accepted for publication in A&