Late-Time Photometry of Type Ia Supernova SN 2012cg Reveals the
  Radioactive Decay of $^{57}$Co
  Seitenzahl et al. (2009) 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 $^{57}{\rm Co}~\to~^{57}{\rm Fe}$, instead of positrons from the
decay chain $^{56}{\rm Co}~\to~^{56}{\rm Fe}$ that dominates the SN light at
earlier times. Using the {\it 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
$^{57}$Co, offering evidence that $^{57}$Co is produced in SN Ia explosions.
However, the data are also consistent with a light echo $\sim14$ mag fainter
than 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 any
SN Ia explosion model, is $0.043^{+0.012}_{-0.011}$, roughly twice Solar. In
the context of current explosion models, this value favors a progenitor white
dwarf with a mass near the Chandrasekhar limit.
