Constraining Type Ia supernova asymmetry with the gamma-ray escape time-scale

Abstract

We calculate the effects of an asymmetric 56Ni distribution in Type Ia supernova (SN Ia) ejecta on the gamma-ray escape time-scale (t0) that characterizes the late light curve (>40 d after peak) and find the effect is modest compared to other possible variations in ejecta structure. We parametrize asymmetry in the 56Ni distribution and calculate t0 for a grid of SN ejecta models spanning a large volume of the asymmetry parameter space. Themodels have spherical density profiles,while the 56Ni distribution in them has various levels of asymmetry. By placing constraints based on the observational measurement of t0 and other general properties of SN Ia ejecta, we find the range of allowed asymmetry in the 56Ni distribution. Considering these constraints we find that some level of asymmetry in the distribution is not ruled out. However, models with a single ejecta mass and varying 56Ni distributions cannot explain the full range of observed t0 values. This strengthens the claim that both Chandrasekhar mass and sub- Chandrasekhar mass explosions are required to explain the diversity of SN Ia observations.