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

The very nearby Type Ia supernova 2014J in M82 offers a rare opportunity to study the physics of thermonuclear supernovae at extremely late phases (gsim800 days). Using the Hubble Space Telescope, we obtained 6 epochs of high-precision photometry for SN 2014J from 277 days to 1181 days past the B-band maximum light. The reprocessing of electrons and X-rays emitted by the radioactive decay chain ${}^{57}\mathrm{Co}\to {}^{57}\mathrm{Fe}$ is needed to explain the significant flattening of both the F606W-band and the pseudo-bolometric light curves. The flattening confirms previous predictions that the late-time evolution of type Ia supernova luminosities requires additional energy input from the decay of 57Co. By assuming the F606W-band luminosity scales with the bolometric luminosity at ~500 days after the B-band maximum light, a mass ratio ${}^{57}\mathrm{Ni}{/}^{56}\mathrm{Ni}\sim {0.065}_{-0.004}^{+0.005}$ is required. This mass ratio is roughly ~3 times the solar ratio and favors a progenitor white dwarf with a mass near the Chandrasekhar limit. A similar fit using the constructed pseudo-bolometric luminosity gives a mass ratio ${}^{57}\mathrm{Ni}{/}^{56}\mathrm{Ni}\sim {0.066}_{-0.008}^{+0.009}$. Astrometric tests based on the multi-epoch HST ACS/WFC images reveal no significant circumstellar light echoes in between 0.3 and 100 pc from the supernova.