What produced the ultraluminous supernova remnant in NGC 6946?

The ultraluminous supernova remnant (SNR) in NGC 6946 is the brightest know n SNR in X-rays, about 1000 times brighter than Cas A. To probe the nature of this remnant and its progenitor, we have obtained high-dispersion optica l echelle spectra. The echelle spectra detect H alpha, [N II], and [O III] lines and resolve these lines into a narrow (FWHM similar to 20-40 km s(-1) ) component from unshocked material and a broad (FWHM similar to 250 km s(- 1)) component from shocked material. Both narrow and broad components have unusually high [N II]/H alpha ratios, about 1. Using the echelle observatio n, archival Hubble Space Telescope images, and archival ROSAT X-ray observa tions, we conclude that the SNR was produced by a normal supernova whose pr ogenitor 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 expand ing into a dense, nitrogen-rich circumstellar nebula created by the progeni tor.