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

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.