What produced the ultraluminous supernova remnant in NGC 6946?

The ultraluminous supernova remnant (SNR) in NGC 6946 is the brightest known 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 optical 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 observation, archival Hubble Space Telescope 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.