  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.
