We present a detection in pre-explosion Hubble Space Telescope (HST) imaging of a point source consistent with being the progenitor star of the Type IIb supernova (SN IIb) 2016gkg. Post-explosion imaging from the Keck Adaptive Optics system was used to perform relative astrometry between the Keck and HST imaging. We identify a single point source in the HST images coincident with the SN position to 0.89-sigma. The HST photometry is consistent with the progenitor star being an A0Ia star with T=9500 K and log (L/Lsun)=5.15. We find that the SN 2016gkg progenitor star appears more consistent with binary than single-star evolutionary models. In addition, early-time light curve data from SN 2016gkg revealed a rapid rise in luminosity within ~0.4 days of non-detection limits, consistent with models of the cooling phase after shock break-out. We use these data to determine an explosion date of 20.15 September 2016 and progenitor star radius of log (R/Rsun)=2.41, which agrees with photometry from the progenitor star. Our findings are also consistent with detections of other SNe IIb progenitor stars, although more luminous and bluer than most other examples.
@article{arxiv.1610.04587,
title = {On the Progenitor of the Type IIb Supernova 2016gkg},
author = {Charles D. Kilpatrick and Ryan J. Foley and Louis E. Abramson and Yen-Chen Pan and Cicero-Xinyu Lu and Peter Williams and Tommaso Treu and Matthew R. Siebert and Christopher D. Fassnacht and Claire E. Max},
journal= {arXiv preprint arXiv:1610.04587},
year = {2017}
}