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 an SN explosion. We present the results from the first 4 yr of survey data, during which these galaxies were observed to produce three 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 a parts per thousand yen 10(4) L-aS (TM) from the first to the last observation. We also search for the low luminosity, long duration transients predicted by Lovegrove & Woosley (2013) for failed explosions of red supergiants. After analysing the first 4 yr 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-aS (TM), a mass range likely associated with failed SNe and, if real, implies that failed SN represents a median fraction of f a parts per thousand integral 0.30 of core collapses, with symmetric 90 per cent confidence limits of 0.07 a parts per thousand currency sign f a parts per thousand currency sign 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 per cent confidence. As the duration of the survey continues to increase, it will begin to constrain the f a parts per thousand integral 10-30 per cent failure rates needed to explain the deficit of massive SN progenitors and the observed black hole mass function.