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Using the Hubble Space Telescope (HST) and the Large Binocular Telescope, wefollowed the evolution of the Type Ia supernova (SN Ia) 2011fe for anunprecedented 1840 days past $B$-band maximum light and over a factor of $7$million in flux. At 1840 days, the 4000 - 17000 \AA{} quasi-bolometricluminosity is just ($420 \pm 20$) $L_{\odot}$. By measuring the late-timequasi-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 $\log(^{57}\mathrm{Co}/^{56}\mathrm{Co}) = -1.59^{+ 0.06 }_{- 0.07 }$. We do nothave a clean detection of $^{55}\mathrm{Fe}$, but find a limit of$^{55}\mathrm{Fe}/^{57}\mathrm{Co} < 0.22$ with 99% confidence. These abundanceratios provide unique constraints on the progenitor system because the centraldensity of the exploding white dwarf(s) dictates these nucleosynthetic yields.The observed ratios strongly prefer the lower central densities ofdouble-degenerate models ($^{55}\mathrm{Fe}/^{57}\mathrm{Co} = 0.27$) over thehigher central densities of near-Chandrasekhar-mass single-degenerate models($^{55}\mathrm{Fe}/^{57}\mathrm{Co} = 0.68$). However, additional theoreticalstudies predicting isotopic yields from a broader range of progenitor systemsare motivated by these unique observations. We will continue to observe SN2011fe for another $\sim 600$ days with HST and possibly beyond.

Whimper of a Bang: Documenting the Final Days of the Nearby Type Ia Supernova 2011fe