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 ^ { 57 } { Co } ~ { } \to~ { } ^ { 57 } { Fe } , instead of positrons from the decay chain ^ { 56 } { Co } ~ { } \to~ { } ^ { 56 } { Fe } 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 ^ { 57 } Co , offering evidence that ^ { 57 } Co is produced in SN Ia explosions .
However , the data are also consistent with a light echo \sim 14 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 .