2015ApJS..221...22I

The main progenitor candidates of Type Ia supernovae (SNe Ia) are white dwarfs in binary systems where the companion star is another white dwarf (double degenerate (DD) system) or a less-evolved, non-degenerate star with R_*≳ 0.1 R_☉(single degenerate system). However, no direct observational evidence exists to tell us which progenitor system is more common. Recent studies suggest that the light curve of a supernova shortly after its explosion can be used to set a limit on the progenitor size, R_*. Here, we report high-cadence monitoring observations of SN 2015F, a normal SN Ia in the galaxy NGC 2442, starting about 84 days before the first light time. Using our daily cadence data, we capture the emergence of the radioactively powered light curve; more importantly, with >97.4% confidence, we detect possible dim precursor emission that appears roughly 1.5 days before the rise of the radioactively powered emission. The signal is consistent with theoretical expectations for a progenitor system involving a companion star with R_*~= 0.1-1 R_☉ or a prompt explosion of a DD system, but is inconsistent with the typically invoked size of a white dwarf progenitor of R_*∼ 0.01 R_☉. Upper limits on the precursor emission also constrain the progenitor size to be R_*≲ 0.1 R_☉with a companion star size of R_*≲ 1.0 R_☉, excluding a very large companion star in the progenitor system. Additionally, we find that the distance to SN 2015F is 23.9±0.4 Mpc.