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

TL;DR

This study presents late-time observations of SN 2012cg that provide evidence for the radioactive decay of $^{57}$Co, offering insights into the nucleosynthesis and progenitor models of Type Ia supernovae.

Contribution

First detection of $^{57}$Co decay signature in a Type Ia supernova at late times, constraining explosion models and progenitor white dwarf mass.

Findings

Evidence for $^{57}$Co decay in SN 2012cg's light curve.

Estimated $^{57}$Ni to $^{56}$Ni mass ratio of approximately 0.043.

Results favor a near-Chandrasekhar mass white dwarf progenitor.

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 $^{57}{\rm Co}~\to~^{57}{\rm Fe}$, instead of positrons from the decay chain $^{56}{\rm Co}~\to~^{56}{\rm Fe}$ 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 $\sim14$ 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 $0.043^{+0.012}_{-0.011}$, roughly twice Solar. In the context of current explosion models, this value favors a progenitor white dwarf with a mass near the Chandrasekhar limit.