LATE-TIME PHOTOMETRY OF TYPE IA SUPERNOVA SN 2012cg REVEALS THE RADIOACTIVE DECAY OF 57Co

Seitenzahl et al. have predicted that roughly three years after its explosion, the light we receive from a Type Ia supernova (SN Ia) will come mostly from reprocessing of electrons and X-rays emitted by the radioactive decay chain Co-57 -> Fe-57, instead of positrons from the decay chain Co-56 -> Fe-56 that dominates the SN light at earlier times. Using the Hubble Space Telescope, we followed the light curve of the SN Ia SN 2012cg out to 1055 days after maximum light. Our measurements are consistent with the light curves predicted by the contribution of energy from the reprocessing of electrons and X-rays emitted by the decay of Co-57, offering evidence that Co-57 is produced in SN Ia explosions. However, the data are also consistent with a light echo similar to 14 mag fainter than SN 2012cg at peak. Assuming no light-echo contamination, the mass ratio of Ni-57 and Ni-56 produced by the explosion, a strong constraint on any SN Ia explosion models, is 0.043(-0.011)(+0.012), roughly twice Solar. In the context of current explosion models, this value favors a progenitor white dwarf with a mass near the Chandrasekhar limit.