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

TL;DR

This study confirms the disappearance of a red supergiant star likely undergoing a failed supernova, supporting the hypothesis that some massive stars end their lives by collapsing into black holes without a bright explosion.

Contribution

First direct confirmation of a failed supernova through optical disappearance and multi-wavelength observations, providing evidence for black hole formation without a typical supernova explosion.

Findings

The star's optical brightness decreased by at least 5 magnitudes.

Mid-IR flux has steadily declined to the lowest levels since 2004.

Late-time emission suggests fallback accretion onto a black hole.

Abstract

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.