Signatures of $^{56}$Ni Mixing and Neutron-rich Ejecta in Supernovae

Abstract

Supernova lightcurves are often interpreted with one-zone radioactive-decay models that ignore a key variable that can affect interpretation and inferred parameters: the distribution of radioactive material. Using a multi-shell model, we explore the impact of $^{56}$Ni mixing in supernovae and r-process material in collapsars. Moving $^{56}$Ni outward reduces the overlying diffusion column, producing faster and brighter rises at fixed $M_{\rm Ni}$, $M_{\rm ej}$, and $E_{\rm k}$, and changes the tail through local gamma-ray leakage. A fast, bright rise is not, by itself, evidence for low ejecta mass or a requirement for engine power, with significant overlap between highly mixed and engine-powered lightcurves. One-zone fits to mixed bolometric light curves produce visually good fits but biased parameters. At fixed opacity, outward mixing is absorbed mainly by low inferred $M_{\rm ej}$ and high inferred $f_{\rm Ni}$, while $M_{\rm Ni}$ remains stable. If opacity is free, the fully mixed case is recovered with $\kappa^{\rm fit}/\kappa^{\rm input}\simeq0.24$. These shifts affect inferred explosion energies and progenitor mappings, and amplified in photometric fits. Exploring collapsar r-process enrichment, we find that the signature is not always a NIR excess and depends sensitively on the nickel-powered background, radial placement, angular distribution, and viewing angle of neutron-rich ejecta. In our setup, spherical models often show optical suppression and delayed colour evolution. Our disk-wind models suggest that fast-rising on-axis GRB-SNe are poor r-process targets for equatorially confined neutron-rich winds, and become constraining only if the r-process material reaches latitudes $\gtrsim 30^\circ$ from the equatorial plane.