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The very nearby Type Ia supernova 2014J in M82 offers a rare opportunity tostudy the physics of thermonuclear supernovae at extremely late phases($\gtrsim$800 days). Using the Hubble Space Telescope (HST), we obtained sixepochs of high precision photometry for SN 2014J from 277 days to 1181 dayspast the $B-$band maximum light. The reprocessing of electrons and X-raysemitted by the radioactive decay chain $^{57}$Co$\rightarrow ^{57}$Fe areneeded to explain the significant flattening of both the $F606W$-band and thepseudo-bolometric light curves. The flattening confirms previous predictionsthat the late-time evolution of type Ia supernova luminosities requiresadditional energy input from the decay of $^{57}$Co (Seitenzahl et al. 2009).By assuming the $F606W$-band luminosity scales with the bolometric luminosityat $\sim$500 days after the $B-$band maximum light, a mass ratio$^{57}$Ni/$^{56}$Ni$\sim$0.065$_{-0.004}^{+0.005}$ is required. This mass ratiois roughly $\sim$3 times the solar ratio and favors a progenitor white dwarfwith a mass near the Chandrasekhar limit. A similar fit using the constructedpseudo-bolometric luminosity gives a mass ratio$^{57}$Ni/$^{56}$Ni$\sim$0.066$_{-0.008}^{+0.009}$. Astrometric tests based onthe multi-epoch HST ACS/WFC images reveal no significant circumstellar lightechoes in between 0.3 pc and 100 pc (Yang et al. 2017) from the supernova.

Late-time Flattening of Type Ia Supernova Light Curves: Constraints From SN 2014J in M82