Detection of an optical filament in the Monogem Ring
  The Monogem Ring is a huge bright soft X-ray enhancement with a diameter of ~ 25$\degr$. 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 POSS II R film copy, at the south-eastern borders of the MR. Spectroscopy revealed this filament to have a very large [SII]$\lambda$ 6716+6731/Halpha ratio of up to ~ 1.8, indicating shock excitation, and a low density of N_e <100 cm^{-3}. There is no hint of [OIII] emission in the spectra. On deep wide-field direct images in Halpha and in [SII] the nebula appears as a ~ 20 arcmin long, thin (~ 1 arcmin), 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. 
R. Weinberger (1), S. Temporin (1,2), B. Stecklum (3) ((1) Institute   of Astrophysics, University of Innsbruck; (2) INAF-Br...