THE OPTICAL COUNTERPART TO THE LUMINOUS X-RAY SUPERNOVA REMNANT IN NGC-6946

We identify a high surface brightness optical supernova remnant (SNR) in NGC 6946 that is coincident with a bright soft X-ray point source detected in a ROSAT PSPC image of this galaxy (Schlegel 1994). Optical CCD/interference filter images and spectra show the object to have an observed Halpha flux of 1.9 x 10(-14) ergs cm-2 s-1, corresponding to an intrinsic Halpha luminosity of 2 x 10(38) (d/5.1 Mpc)2 ergs s-1. A moderate-resolution optical spectrum shows normal SNR line emissions, with no high velocities (V < 400 km s-1) or relative line strengths indicative of enriched heavy element abundances. This suggests a relatively old SNR, a conclusion supported by the presence of an emission nebula at its position on photographic plates dating back to at least 1921. With both an optical line flux and soft X-ray flux exceeding those of other luminous extragalactic SNRs, this object may have an optical luminosity near the maximum limit for SNRs. Assuming current flux levels, we estimate the SNR's age to be less-than-or-equal-to 3500 yr and suggest its unusual luminosity is due to both its expansion into dense surroundings and a relatively low column density along our line of sight.