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

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 $M_{\mathrm{N}\mathrm{i}}$, $M_{\mathrm{e}\mathrm{j}}$, and $E_{\mathrm{k}}$, 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 $M_{\mathrm{e}\mathrm{j}}$ and high inferred $f_{\mathrm{N}\mathrm{i}}$, while $M_{\mathrm{N}\mathrm{i}}$ remains stable. If opacity is free, the fully mixed case is recovered with ${\kappa }^{\mathrm{f}\mathrm{i}\mathrm{t}}\mathrm{/}{\kappa }^{\mathrm{i}\mathrm{n}\mathrm{p}\mathrm{u}\mathrm{t}}\simeq 0.24$. 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.