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, Kochanek & Stanek. This ∼25 M⊙ red supergiant experienced a weak ∼10^6 L⊙ optical outburst in 2009 and is now at least 5 mag 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⊙) 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 ∼t^(−4/3). We conclude that the transient is unlikely to be an 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 on to a newly formed black hole. Future IR and X-ray observations are needed to confirm this interpretation of the fate for the star.
© 2017 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2017 March 29. Received 2017 March 28; in original form 2016 September 4. Published: 01 April 2017. We thank Ron Arbour for sharing his images of NGC 6946. We thank Klaas Wiersema and Andrew Levan for identifying observations covering N6946-BH1 in the INT archive. We thank John Beacom and the anonymous referee for helpful comments on the manuscript. Financial support for this work was provided by the NSF through grant AST-1515876. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA, and in part on observations made with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program GO-14266. This work is based in part on observations made with the Large Binocular Telescope (LBT). The LBT is an international collaboration among institutions in the USA, Italy and Germany. The LBT Corporation partners are: The University of Arizona on behalf of the Arizona University System; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max Planck Society, the Astrophysical Institute Potsdam and Heidelberg University; The Ohio State University; The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota and University of Virginia. This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency.
Submitted - 1609.01283v1.pdf
Published - stx816.pdf