2 00 1 Late Light Curves of Type Ia Supernovae

We extend earlier efforts to determine whether the late (t≥60d) 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 pho-tometry 35 days or more after maximum light. Assuming all SNe within a given ∆m 15 (B) range form a distinct sub-class, 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 non-local and time-dependent energy deposition produces a correction of as much as ≤0.10 m for Chandrasekhar models and ≤0.18 m for sub-Chandrasekhar models. We find that applying a filter efficiency correction, derived from measured spectra, to B, V, R, and I light-curves after day 50 can produce a consistent bolometric light-curve. The V band is an accurate indicator of total emission in the 3500Å-9700Å 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. Varying bolometric corrections for the other bands are derived. We see significant evolution of the colors of SNe Ia between day 50 and day 170. We suggest that this may be due to the transition from spectra dominated by emission lines from the radioactive nucleus, 56 Co, to those from the stable daughter nucleus, 56 Fe. We show that the B, V, R, and I band light-curves of SNe Ia after t≥60d can be completely explained with energy deposition from 56 Co decay photons and positrons if substantial positron escape occurs.