The search for failed supernovae with the Large Binocular Telescope:
  confirmation of a disappearing star
  We present Hubble Space Telescope imaging confirming the optical
disappearance of the failed supernova (SN) candidate identified by Gerke et al.
(2015). This $\sim 25~M_{\odot}$ red supergiant experienced a weak $\sim
10^{6}~L_{\odot}$ optical outburst in 2009 and is now at least 5 magnitudes
fainter than the progenitor in the optical. The mid-IR flux has slowly
decreased to the lowest levels since the first measurements in 2004. There is
faint ($2000-3000~L_{\odot}$) near-IR emission likely associated with the
source. We find the late-time evolution of the source to be inconsistent with
obscuration from an ejected, dusty shell. Models of the spectral energy
distribution indicate that the remaining bolometric luminosity is $>6$ times
fainter than that of the progenitor and is decreasing as $\sim t^{-4/3}$. We
conclude that the transient is unlikely to be a SN impostor or stellar merger.
The event is consistent with the ejection of the envelope of a red supergiant
in a failed SN and the late-time emission could be powered by fallback
accretion onto a newly-formed black hole. Future IR and X-ray observations are
needed to confirm this interpretation of the fate for the star.
