The Search for Failed Supernovae with The Large Binocular Telescope: First Candidates

We are monitoring 27 galaxies within 10 Mpc using the Large Binocular Telescope to search for failed supernovae (SNe), massive stars that collapse to form a black hole without a SN explosion. We present the results from the first 4 years of survey data, during which these galaxies were observed to produce 3 successful core-collapse SNe. We search for stars that have "vanished" over the course of our survey, by examining all stars showing a decrease in luminosity of $\Delta \nu L_{\nu} \ge 10^4L_{\odot}$ from the first to the last observation. We also search for the low luminosity, long duration transients predicted by \citet{lovegrove2013} for failed explosions of red supergiants. After analyzing the first 4 years of data in this first direct search for failed SNe, we are left with one candidate requiring further study. This candidate has an estimated mass of 18-25$M_{\odot}$, a mass range likely associated with failed SNe and, if real, implies that failed SN represent a median fraction of $f \simeq 0.30$ of core-collapses, with symmetric 90% confidence limits of $0.07 \le f \le 0.62$. If follow up data eliminate this candidate, we find an upper limit on the fraction of core collapses leading to a failed SN of $f<0.40$ at 90% confidence. As the duration of the survey continues to increase, it will begin to constrain the $f \simeq$ 10-30% failure rates needed to explain the deficit of massive SN progenitors and the observed black hole mass function.