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Seitenzahl et al. (2009) have predicted that roughly three years after itsexplosion, the light we receive from a Type Ia supernova (SN Ia) will comemostly from reprocessing of electrons and X-rays emitted by the radioactivedecay chain $^{57}{\rm Co}~\to~^{57}{\rm Fe}$, instead of positrons from thedecay chain $^{56}{\rm Co}~\to~^{56}{\rm Fe}$ that dominates the SN light atearlier times. Using the {\it Hubble Space Telescope}, we followed the lightcurve of the SN Ia SN 2012cg out to $1055$ days after maximum light. Ourmeasurements are consistent with the light curves predicted by the contributionof energy from the reprocessing of electrons and X-rays emitted by the decay of$^{57}$Co, offering evidence that $^{57}$Co is produced in SN Ia explosions.However, the data are also consistent with a light echo $\sim14$ mag fainterthan SN 2012cg at peak. Assuming no light-echo contamination, the mass ratio of$^{57}$Ni and $^{56}$Ni produced by the explosion, a strong constraint on anySN Ia explosion model, is $0.043^{+0.012}_{-0.011}$, roughly twice Solar. Inthe context of current explosion models, this value favors a progenitor whitedwarf with a mass near the Chandrasekhar limit.

Late-Time Photometry of Type Ia Supernova SN 2012cg Reveals the Radioactive Decay of $^{57}$Co