UBVR and IR photometry of SN 2011fe (Ia)

At 1840 days, the 4000-17000{AA} quasi-bolometric luminosity is just (420+/-20)L_{sun}_. By measuring the late-time quasi-bolometric light curve, we present the first confident detection of ^57^Co decay in a SN Ia light curve and estimate a mass ratio of log(^57^Co/^56^Co)=-1.59_-0.07_^+0.06^.

We do not have a clean detection of ^55^Fe, but find a limit of ^55^Fe/^57^Co<0.22 with 99% confidence. These abundance ratios provide unique constraints on the progenitor system because the central density of the exploding white dwarf(s) dictates these nucleosynthetic yields. The observed ratios strongly prefer the lower central densities of double-degenerate models (^55^Fe/^57^Co=0.27) over the higher central densities of near-Chandrasekhar-mass single-degenerate models (^55^Fe/^57^Co=0.68).