Keywords

We report initial observations and analysis on the Type IIb SN 2016gkg in the nearby galaxy NGC 613. SN 2016gkg exhibited a clear double-peaked light curve during its early evolution, as evidenced by our intensive photometric follow-up campaign. SN 2016gkg shows strong similarities with other Type IIb SNe, in particular, with respect to the He i emission features observed in both the optical and near-infrared. SN 2016gkg evolved faster than the prototypical Type IIb SN 1993J, with a decline similar to that of SN 2011dh after the first peak. The analysis of archival Hubble Space Telescope images indicate a pre-explosion source at SN 2016gkg's position, suggesting a progenitor star with a ∼mid-F spectral type and initial mass $15\mbox{--}20$ M${}_{\odot }$, depending on the distance modulus adopted for NGC 613. Modeling the temperature evolution within $5\,\mathrm{days}$ of explosion, we obtain a progenitor radius of $\sim 48\mbox{--}124$ R${}_{\odot }$, smaller than that obtained from the analysis of the pre-explosion images ($240\mbox{--}320$ R${}_{\odot }$).