Determining the 56Ni distribution of type Ia supernovae from observations within days of explosion

Recent studies have shown how the distribution of Ni-56 within the ejected material of type Ia supernovae can have profound consequences on the observed light curves. Observations at early times can therefore provide important details on the explosion physics in thermonuclear supernovae, which are poorly constrained. To this end, we present a series of radiative transfer calculations that explore variations in the Ni-56 distribution. Our models also show the importance of the density profile in shaping the light curve, which is often neglected in the literature. Using our model set, we investigate the observations that are necessary to determine the Ni-56 distribution as robustly as possible within the current model set. We find that this includes observations beginning at least similar to 14 days before B-band maximum, extending to approximately maximum light with a relatively high (less than or similar to 3 day) cadence, and in at least one blue and one red band (such as B and R, or g and r) are required. We compare a number of well-observed type Ia supernovae that meet these criteria to our models and find that the light curves of similar to 70-80% of objects in our sample are consistent with being produced solely by variations in the Ni-56 distributions. The remaining supernovae show an excess of flux at early times, indicating missing physics that is not accounted for within our model set, such as an interaction or the presence of short-lived radioactive isotopes. Comparing our model light curves and spectra to observations and delayed detonation models demonstrates that while a somewhat extended Ni-56 distribution is necessary to reproduce the observed light curve shape, this does not negatively affect the spectra at maximum light. Investigating current explosion models shows that observations typically require a shallower decrease in the Ni-56 mass towards the outer ejecta than is produced for models of a given Ni-56 mass. Future models that test differences in the explosion physics and detonation criteria should be explored to determine the conditions necessary to reproduce the Ni-56 distributions found here.