We report evidence for excess blue light from the Type Ia supernova SN 2012cg at fifteen and sixteen 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 M _ { B } = -19.62 \pm 0.02 mag and \Delta m _ { 15 } ( B ) = 0.86 \pm 0.02 .
Our data set is extensive , with photometry in 7 filters from 5 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 ( 44 ) favors a main-sequence companion of about 6 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 .