Direct Detection of an Ultraluminous Ultraviolet Source

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.

4 pages, accepted for ApJ Letters

References in corpus (2)

• The Long-Term Variability of the X-Ray Sources in NGC 6946 and NGC 4485/4490
• Optical Spectroscopy of the ULX-Associated Nebula MF16

Cited by in corpus (25)

• Ultraluminous X-ray Sources in the Chandra and XMM-Newton Era
• A spectral-timing model for ULXs in the super-critical regime
• Ultraluminous X-ray sources
• Periodic Fast Radio Bursts from Luminous X-ray Binaries
• Compact Optical Counterparts of Ultraluminous X-ray Sources
• Ultraluminous X-Ray Sources
• Optically thick outflows in ultraluminous supersoft sources
• Optical emission of the ultraluminous X-ray source NGC 5408 X-1: donor star or irradiated accretion disk?
• Swift observations of V404 Cyg during the 2015 outburst: X-ray outflows from super-Eddington accretion
• Revisiting the ultraluminous supersoft source in M101: an optically thick outflow model
• Detection of Strong Short-Term Variability in NGC 6946 X-1
• Multiband counterparts of two eclipsing ultraluminous X-ray sources in M 51
• The Nature of the UV/optical Emission of the Ultraluminous X-Ray Source in Holmberg II
• A New, Larger Sample of Supernova Remnants in NGC 6946
• The Optical Counterpart of NGC 1313 X-1
• Ultra-luminous X-ray Sources as Supercritical Accretion Disks: Spectral Energy Distributions
• MUSE spectroscopy of the ULX NGC 1313 X-1: a shock-ionised bubble, an X-ray photoionised nebula, and two supernova remnants
• A broadband look at the old and new ULXs of NGC 6946
• Spitzer Observations of MF 16 Nebula and the associated Ultraluminous X-ray Source
• The Local Environment of Ultraluminous X-ray Sources Viewed by XMM Newton's Optical Monitor
• The Supernova Remnant Population of NGC6946 as Observed in [Fe II] 1.644 $\mu$m with HST
• Paving the way to simultaneous multi-wavelength astronomy
• Detection of 125.5-day Optical Periodic Modulation of the Neutron Star M51 ULX-8
• Simultaneous NICER and NuSTAR observations of the Ultraluminous source NGC 4190 ULX-1
• Long-Term X-Ray/UV Variability in ULXs