Late Light Curves of Type Ia Supernovae

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Abstract
We extend earlier efforts to determine whether the late (t ≥ 60 days) light curves of Type Ia SNe are better explained by the escape of positrons from the ejecta or by the complete deposition of positron kinetic energy in a trapping magnetic field. We refine our selection of Ia SNe, using those that have extensive BVRI photometry 35 days or more after maximum light. Assuming that all SNe within a given Δm15(B) range form a distinct subclass, we fit a combined light curve for all class members with a variety of models. We improve our previous calculations of energy deposition rates by including the transport of the Comptonized electrons. Their nonlocal and time-dependent energy deposition produces a correction of as much as 0.10 mag for Chandrasekhar-mass models and 0.18 mag for sub-Chandrasekhar-mass models. We produce bolometric corrections, derived from measured spectra, to B, V, R, and I light curves after day 50. Comparisons of the resulting bolometric light curves with simulated energy deposition rates demonstrate that the energy deposition from the photons and positrons created in 56Co → 56Fe decays are consistent with the observations if positron escape is assumed. This implies that there is no evidence of additional sources of energy deposition or of a shift of emission into unobserved wavelength ranges between days 60 and 900. The V band is shown to be an accurate indicator of total emission in the 3500-9700 Å range, with a constant fraction (~25%) appearing in the V band after day 50. This suggests that the V band scales with the bolometric luminosity and that the deposited energy is instantaneously recycled into optical emission during this epoch. We see significant evolution of the colors of SNe Ia between days 50 and 170. We suggest that this may be due to the transition from spectra dominated by emission lines from the radioactive nucleus, 56Co, to those from the stable daughter nucleus, 56Fe.
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