Detection of an optical filament in the Monogem Ring

¹ Institut für Astrophysik, Universität Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria e-mail: [ronald.weinberger;giovanna.temporin]@uibk.ac.at
² Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany e-mail: stecklum@tls-tautenburg.de

Received: 9 September 2005
Accepted: 17 November 2005

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  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 []λ 6716+6731/Hα ratio of up to ~1.8, indicating shock excitation, and a low density of N_e < 100 cm^-3. There is no hint of [] emission in the spectra. On deep wide-field direct images in Hα and in [] 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.