The search for failed supernovae with the Large Binocular Telescope: a new candidate and the failed SN fraction with 11 yr of data

Abstract: We present updated results of the Large Binocular Telescope Search for Failed Supernovae. This search monitors luminous stars in 27 nearby galaxies with a current baseline of 11~yr of data. We re-discover the failed supernova (SN) candidate N6946-BH1 as well as a new candidate, M101-OC1. M101-OC1 is a blue supergiant that rapidly disappears in optical wavelengths with no evidence for significant obscuration by warm dust. While we consider other options, a good explanation for the fading of M101-OC1 is a failed SN, but follow-up observations are needed to confirm this. Assuming only one clearly detected failed SN, we find a failed SN fraction $f = 0.16<sup>{+0.23}_{-0.12}$ at 90 per~cent confidence. We also report on a collection of stars that show slow ($\sim$decade), large amplitude ($\Delta L/L &gt; 3$) luminosity changes.