Emission line models for the lowest mass core-collapse supernovae -- II. Full 3D NLTE radiative transfer modelling of a $9.0\,M_\odot$ neutrino-driven explosion

The nebular phase of a supernova (SN) occurs several months to years after the explosion, when the ejecta become mostly optically thin yet there still is sufficient radioactive material to keep the supernova bright. The asymmetries created by the explosion are encoded into the line profiles of the emission lines which appear in the nebular phase. In order to make accurate predictions for these line profiles, Non-Local Thermodynamic Equilibrium (NLTE) radiative transfer calculations need to be carried out. In this work, we use \texttt{ExTraSS} (EXplosive TRAnsient Spectral Simulator) -- which was recently upgraded into a full 3D NLTE radiative transfer code (including photoionization and line-by-line transfer effects) -- to carry out such calculations. \texttt{ExTraSS} is applied to a 3D explosion model of a $9.0\,M_\odot$ H-rich progenitor which is evolved into the homologous phase. Synthetic spectra are computed and the lines from different elements are studied for varying viewing angles. The model spectra are also compared against observations of SN 1997D and SN 2016bkv. The model is capable of creating good line profile matches for both SNe, and reasonable luminosity matches for He, C, O, and Mg lines for SN 1997D -- however H$\alpha$ and Fe I lines are too strong.