Early Lightcurves of Type Ia Supernovae are Consistent with Nondegenerate Progenitor Companions

Abstract: If Type Ia supernovae (SNe~Ia) result from a white dwarf being ignited by Roche lobe overflow from a nondegenerate companion, then as the supernova explosion runs into the companion star its ejecta will be shocked, causing an early blue excess in the lightcurve. A handful of these excesses have been found in single-object studies, but inferences about the population of SNe~Ia as a whole have been limited because of the rarity of multiwavelength followup within days of explosion. Here we present a three-year investigation yielding an unbiased sample of nine nearby ($z<0.01$) SNe~Ia with exemplary early data. The data are truly multiwavelength, covering $UBVgri$ and Swift bandpasses, and also early, with an average first epoch 16.0 days before maximum light. Of the nine objects, three show early blue excesses. We do not find enough statistical evidence to reject the null hypothesis that SNe~Ia predominantly arise from Roche-lobe-overflowing single-degenerate systems ($p=0.94$). When looking at the objects' colors, we find the objects are almost uniformly near-UV-blue, in contrast to earlier literature samples which found that only a third of SNe~Ia are near-UV-blue, and we find a seemingly continuous range of $B-V$ colors in the days after explosion, again in contrast with earlier claims in the literature. This study highlights the importance of early, truly multiwavelength, high-cadence data in determining the progenitor systems of SNe~Ia and in revealing their diverse early behavior.