Optical detection of a fast shock wave associated with the Cygnus Loop

A faint filament which lies 4-6 arcmin outside the bright optical filaments of the NW section of the Cygnus Loop is detected only in hydrogen Balmer emission in the range 4500-6900 A. This unusual nebular spectrum is similar to that of the optical filaments of Tycho's Supernova Remnant. Such a spectrum is interpreted as emission from neutral gas swept up by a fast shock. The discovery of this filament supports the fast shock interpretation of the high-velocity H-alpha observed in the Cygnus Loop and in other old supernova remnants. Implications for models of the Cygnus Loop and the interstellar medium in general are discussed.