Direct Detection of an Ultraluminous Ultraviolet Source

TL;DR

This study reports the first direct detection of an ultraluminous ultraviolet source associated with an ultraluminous X-ray source in NGC 6946, using Hubble Space Telescope observations revealing a point source and nebula in the far UV.

Contribution

It provides the first direct detection and characterization of an ultraluminous UV source linked to an X-ray source, proposing models for its emission spectrum.

Findings

Detected a point-like UV source coincident with the X-ray source

The UV flux is consistent with a ~30,000 K blackbody, possibly from a massive star

A multicolor disk blackbody model fits the data and explains ionizing radiation

Abstract

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.