LATE-TIME PHOTOMETRY OF TYPE IA SUPERNOVA SN 2012cg REVEALS THE RADIOACTIVE DECAY OF ⁵⁷Co

Abstract

Seitenzahl et al. 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 Co→ 57 Fe, instead of positrons from the decay chain 56 Co→ 56 Fe that dominates the SN light at earlier times. Using the 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 ∼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 models, 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.

Constraining the progenitor of the Type Ia Supernova SN 2012cg

“…For a comprehensive discussion, the results from other studies such as the deep radio observations of Chomiuk et al (2015), the detection of blueshifted Na i D (Maguire et al 2013) and the late-time photometry observations (Graur et al 2016) for SN 2012cg are also included in this table.…”

“…To place strict constraints on the properties of CSM and the observable signatures of the SN shock interacting with the CSM, detailed numerical hydrodynamical simulations for the interaction between the blast wave of the SN and CSM/ISM are required, coupled with different explosion models and the detailed pre-SN massloss history, i.e., different configurations of outflow material around progenitor systems. In addition, (Graur et al 2016) suggested that the decay of 57 Co and 55 Fe produced from current modelling for the two CO WDs merger model have difficulties in explaining the slow-down decline light curve of SN 2012cg at late times. How- ever, they also pointed out that a similar behavior of the late-time light curve of SN 2012cg can be explained by a light echo, which requires future follow-up observations for further confirmation.…”

“…By analyzing pre-explosion archival WFPC2 images of NGC 4424 in F606W and F814W band from the Hubble Space Telescope (HST), Graur et al (2016) found no source within a 2" radius of the location of SN 2012cg down to limits of M V ≈ −6.0 and M V ≈ −5.4 mags, and thus excluded most supergiants as potential binary companions of the progenitor of SN 2012cg. By taking into account Galactic and host-galaxy extinctions for these filters, they place an upper-limit on the luminosity of pre-explosion progenitor of SN 2012cg of < 840 L ⊙ .…”

“…If the spin-down timescale is longer than about 10 6 yrs, the CSM around the progenitor system could become diffuse and reach a density similar to that of the ISM, leading to the lack of radio and X-ray emission which are absent in SN 2012cg (Chomiuk et al 2015). Also, the MS or RG companion star might shrink rapidly before the SN Ia explosion occurs by exhausting most of its H-rich envelope during a long spin-down ( 10 8 yrs) phase, explaining the non-detection of a pre-explosion companion star (Graur et al 2016) and the absence of swept-up H in the late-time spectra of SN 2012cg (Maguire et al 2016). However, no (or a very weak) interaction signature from the SN-companion (because of a small companion) or SN-CSM (due to the low density CSM) interaction is predicted in this scenario, making it extremely difficult to explain the early excess UV luminosity seen in SN 2012cg.…”

“…Here, only Heburning donor stars are considered in the Sub-Ch-mass scenario. The gray dashed line gives an upper-limit on the progenitor luminosity of SN 2012cg from HST pre-explosion observations (Graur et al 2016). …”