Observations of SN 2015F Suggest a Correlation between the Intrinsic Luminosity of Type Ia Supernovae and the Shape of Their Light Curves >900 Days after Explosion

The late-time light curves of Type Ia supernovae (SNe Ia), observed >900 days after explosion, present the possibility of a new diagnostic for SN Ia progenitor and explosion models. First, however, we must discover what physical process (or processes) leads to the slow-down of the light curve relative to a pure ⁵⁶Co decay, as observed in SNe 2011fe, 2012cg, and 2014J. We present Hubble Space Telescope observations of SN 2015F, taken ≈600-1040 days past maximum light. Unlike those of the three other SNe Ia, the light curve of SN 2015F remains consistent with being powered solely by the radioactive decay of ⁵⁶Co. We fit the light curves of these four SNe Ia in a consistent manner and measure possible correlations between the light-curve stretch - a proxy for the intrinsic luminosity of the SN - and the parameters of the physical model used in the fit. We propose a new, late-time Phillips-like correlation between the stretch of the SNe and the shape of their late-time light curves, which we parameterize as the difference between their pseudo-bolometric luminosities at 600 and 900 days: ΔL ₉₀₀ = log(L ₆₀₀/L ₉₀₀). Our analysis is based on only four SNe, so a larger sample is required to test the validity of this correlation. If true, this model-independent correlation provides a new way to test which physical process lies behind the slow-down of SN Ia light curves >900 days after explosion, and, ultimately, fresh constraints on the various SN Ia progenitor and explosion models.