  The ultraluminous supernova remnant (SNR) in NGC 6946 is the brightest known
SNR in X-rays, ~1000 times brighter than Cas A. To probe the nature of this
remnant and its progenitor, we have obtained high-dispersion optical echelle
spectra. The echelle spectra detect H-alpha, [N II], and [O III] lines, and
resolve these lines into a narrow (FWHM ~20--40 km/s) component from un-shocked
material and a broad (FWHM ~250 km/s) component from shocked material. Both
narrow and broad components have unusually high [N II]/H-alpha ratios, ~1.
Using the echelle observation, archival HST images, and archival ROSAT X-ray
observations, we conclude that the SNR was produced by a normal supernova,
whose progenitor was a massive star, either a WN star or a luminous blue
variable. The high luminosity of the remnant is caused by the supernova ejecta
expanding into a dense, nitrogen-rich circumstellar nebula created by the
progenitor.
