The whisper of a whimper of a bang: 2400 d of the Type Ia SN 2011fe reveals the decay of ⁵⁵Fe

Abstract

We analyse new multifilter Hubble Space Telescope (HST) photometry of the normal Type Ia supernova (SN Ia) 2011fe out to ≈2400 d after maximum light, the latest observations to date of a SN Ia. We model the pseudo-bolometric light curve with a simple radioactive decay model and find energy input from both ⁵⁷Co and ⁵⁵Fe are needed to power the late-time luminosity. This is the first detection of ⁵⁵Fe in a SN Ia. We consider potential sources of contamination such as a surviving companion star or delaying the deposition time-scale for ⁵⁶Co positrons but these scenarios are ultimately disfavored. The relative isotopic abundances place direct constraints on the burning conditions experienced by the white dwarf (WD). Additionally, we place a conservative upper limit of <10^-3 M_⊙ on the synthesized mass of ⁴⁴Ti. Only two classes of explosion models are currently consistent with all observations of SN 2011fe: (1) the delayed detonation of a low-ρ_c, near-M_Ch (1.2-1.3 M_⊙) WD, or (2) a sub-M_Ch (1.0-1.1 M_⊙) WD experiencing a thin-shell double detonation.