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We present very early, high-cadence photometric observations of the nearbyType Ia SN 2017cbv. The light curve is unique in that it has a blue bump duringthe first five days of observations in the U, B, and g bands, which is clearlyresolved given our photometric cadence of 5.7 hr during that time span. Wemodel the light curve as the combination of early shocking of the supernovaejecta against a nondegenerate companion star plus a standard SN Ia component.Our best-fit model suggests the presence of a subgiant star 56 solar radii fromthe exploding white dwarf, although this number is highly model-dependent.While this model matches the optical light curve well, it overpredicts theobserved flux in the ultraviolet bands. This may indicate that the shock is nota blackbody, perhaps because of line blanketing in the UV. Alternatively, itcould point to another physical explanation for the optical blue bump, such asinteraction with circumstellar material or an unusual nickel distribution.Early optical spectra of SN 2017cbv show strong carbon absorption up throughday -13 with respect to maximum light, suggesting that the progenitor systemcontains a significant amount of unburned material. These early results on SN2017cbv illustrate the power of early discovery and intense follow-up of nearbysupernovae to resolve standing questions about the progenitor systems andexplosion mechanisms of SNe Ia.

Early Blue Excess from the Type Ia Supernova 2017cbv and Implications for Its Progenitor