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
5E-16 erg s^-1 cm^-2 Ang^-1 and the nebula has a flux of 1.6E-15 erg s^-1 cm^-2
Ang^-1, quoted at 1533 Ang and assuming an extinction of A_V = 1.54. Thus, MF
16 appears to host the first directly detected ultraluminous UV source (ULUV).
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 HeII 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 HeII fluxes and can produce
the needed ionizing radiation. Additional observations are required to
determine the physical nature of the source.Comment: 4 pages, accepted for ApJ Letter
