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We have identified a high surface brightness optical supernova remnant (SNR) in the face-on spiral galaxy NGC~6946 that is coincident with a bright soft X--ray point source detected in a ROSAT PSPC image of this galaxy by Schlegel (1993, ApJ Letters , in press). NGC~6946 has produced six historical supernovae, but this object is not associated directly with any of them. Still, the large X--ray luminosity of the object and high SN rate of NGC~6946 made it seem likely that the object was a very young SNR from a massive star.
Optical CCD/interference filter images from KPNO and spectra from the MDM Observatory in Arizona show the object to have an observed H$\alpha$\ flux of $1.9 \times\ 10^{-14} \:{\rm ergs\:cm^{-2} \: s^{-1}}$, corresponding to an intrinsic H$\alpha$\ luminosity of $2 \times\:10^{38} \:(\rm d/5.1\:Mpc)^{2} \:{\rm ergs\:s^{-1}}$. Our moderate resolution optical spectrum shows normal SNR line emissions, with no high velocities ($V < 400 \:{\rm km\:s^{-1}}$) 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 this position on photographic plates dating back to at least 1921 (and probably 1899). 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. We suggest this large luminosity is due to both its expansion into dense surroundings and a relatively low column density along our line of sight. Assuming current flux levels, we estimate the SNR's age to be $\leq$ 3500 yrs.
This work has been supported by the Johns Hopkins University Center for Astrophysical Sciences and by a Burke Research Grant from Dartmouth College.
