Direct Detection of an Ultraluminous Ultraviolet Source

We present Hubble Space Telescope observations in the far UV of the ultraluminous X-ray source in NGC 6946 associated with the optical nebula MF 16. Both a point-like source coincident with the X-ray source and the surrounding nebula are detected in the FUV. The point source has a flux of 5 × 10^-16 erg s^-1 cm^-2 Å^-1, and the nebula has a flux of 1.6 × 10^-15 erg s^-1 cm^-2 Å^-1, quoted at 1533 Å and assuming an extinction of A_V = 1.54. Thus, MF 16 appears to host the first directly detected ultraluminous UV source. The flux of the point-like source is consistent with a blackbody with T≈ 30,000 K, possibly from a massive companion star, but this spectrum does not create sufficient ionizing radiation to produce the nebular He II flux, and a second, hotter emission component would be required. A multicolor disk blackbody spectrum truncated with an outer disk temperature of ~16,000 K provides an adequate fit to the FUV, B, V, I, and He II fluxes and can produce the needed ionizing radiation. Additional observations are required to determine the physical nature of the source.