Positron Escape from Type Ia Supernovae

We generate bolometric light curves for a variety of Type Ia supernova models at late times, simulating gamma-ray and positron transport for various assumptions about the magnetic field and ionization of the ejecta. These calculated light curve shapes are compared with light curves of specific supernovae for which there have been adequate late observations. The selection of models is generally not based upon the ability to fit the late observations, but rather because the model has been demonstrated by other authors to approximate the spectra and early light curves of that specific SN. From these comparisons we draw two conclusions: whether a suggested model is an acceptable approximation of a particular event, and, given that it is, the magnetic field characteristics and degree of ionization that are most consistent with the observed light curve shape. For the 10 SNe included in this study, five strongly suggest 56Co positron escape as would be permitted by a weak or radially combed magnetic field. Of the remaining five SNe, none clearly show the upturned light curve expected for positron trapping in a strong, tangled magnetic field. Chandrasekhar mass models can explain normally luminous, slightly subluminous, and superluminous supernova light curves; sub-Chandrasekhar mass models have difficulties with subluminous SNe. An estimate of the galactic positron production rate from Type Ia SNe is compared with gamma-ray observations of Galactic 511 keV annihilation radiation. Additionally, we emphasize the importance of correctly treating the positron transport for calculations of spectra, or any properties, of Type Ia SNe at late epochs (≥200 days).