SN. 2012cg: EVIDENCE FOR INTERACTION BETWEEN A NORMAL SN Ia AND A NON-DEGENERATE BINARY COMPANION

We report evidence for excess blue light from the Type Ia supernova (Sn Ia) SN. 2012cg at 15 and 16 days before maximum B-band brightness. The emission is consistent with predictions for the impact of the supernova on a non-degenerate binary companion. This is the first evidence for emission from a companion to a normal SN. Ia. Sixteen days before maximum light, the B - V color of SN. 2012cg is 0.2 mag bluer than for other normal SN. Ia. At later times, this supernova has a typical SN. Ia light curve, with extinction-corrected MB = -19.62 +/- 0.02 mag and Delta m(15)(B)= 0.86 +/- 0.02. Our data set is extensive, with photometry in seven filters from five independent sources. Early spectra also show the effects of blue light, and high-velocity features are observed at early times. Near maximum, the spectra are normal with a silicon velocity v(Si) = -10,500 km s(-1). Comparing the early data with models by Kasen favors a main-sequence companion of about six solar masses. It is possible that many other SN Ia have main-sequence companions that have eluded detection because the emission from the impact is fleeting and faint.