2006A&A...448.1095W

The Monogem Ring is a huge bright soft X-ray enhancement with a diameter of ∼25deg. 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 HI shell was expected to be detectable. Here we report on the discovery of a very faint arc-like nebula on a POSSII R film copy, at the south-eastern borders of the MR. Spectroscopy revealed this filament to have a very large [SII]λ 6716+6731/Hα ratio of up to ∼1.8, indicating shock excitation, and a low density of N_e<100/cm3. There is no hint of [OIII] emission in the spectra. On deep wide-field direct images in Hα and in [SII] 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.