Late-Time Photometry of Type Ia Supernova SN 2012cg Reveals the Radioactive Decay of ${}^{57}$Co

Abstract

Seitenzahl et al. (2009) 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 ${}^{57}\mathrm{Co}\to {}^{57}\mathrm{Fe}$, instead of positrons from the decay chain ${}^{56}\mathrm{Co}\to {}^{56}\mathrm{Fe}$ that dominates the SN light at earlier times. Using the {\it 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 $0.043_{-0.011}^{+0.012}$, roughly twice Solar. In the context of current explosion models, this value favors a progenitor white dwarf with a mass near the Chandrasekhar limit.

Updated to reflect the final version published by ApJ. For a video about the paper, see https://youtu.be/t3pUbZe8wqk

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