The search for failed supernovae with the Large Binocular Telescope: confirmation of a disappearing star

Abstract

We present Hubble Space Telescope imaging confirming the optical\ndisappearance of the failed supernova (SN) candidate identified by Gerke et al.\n(2015). This $\\sim 25~M_{\\odot}$ red supergiant experienced a weak $\\sim\n10^{6}~L_{\\odot}$ optical outburst in 2009 and is now at least 5 magnitudes\nfainter than the progenitor in the optical. The mid-IR flux has slowly\ndecreased to the lowest levels since the first measurements in 2004. There is\nfaint ($2000-3000~L_{\\odot}$) near-IR emission likely associated with the\nsource. We find the late-time evolution of the source to be inconsistent with\nobscuration from an ejected, dusty shell. Models of the spectral energy\ndistribution indicate that the remaining bolometric luminosity is $>6$ times\nfainter than that of the progenitor and is decreasing as $\\sim t^{-4/3}$. We\nconclude that the transient is unlikely to be a SN impostor or stellar merger.\nThe event is consistent with the ejection of the envelope of a red supergiant\nin a failed SN and the late-time emission could be powered by fallback\naccretion onto a newly-formed black hole. Future IR and X-ray observations are\nneeded to confirm this interpretation of the fate for the star.\n