Modelling supernova nebular lines in 3D with extrass

We present EXplosive TRAnsient Spectral Simulator (EXTRASS), a newly developed code aimed at generating 3D spectra for supernovae in the nebular phase by using modern multidimensional explosion models as input. It is well established that supernovae are asymmetric by nature, and that the morphology is encoded in the line profiles during the nebular phase, months after the explosion. In this work, we use EXTRASS to study one such simulation of a 3.3 M-circle dot He-core explosion (M-ejecta = 1.3 M-circle dot, E-kin = 1.05 x 10(51) erg) modelled with the Prometheus-HotB code and evolved to the homologous phase. Our code calculates the energy deposition from the radioactive decay of 56(Ni) -> Co-56 -> Fe-56 and uses this to determine the Non-Local-Thermodynamic-Equilibrium temperature, excitation, and ionization structure across the nebula. From the physical condition solutions we generate the emissivities to construct spectra depending on viewing angles. Our results show large variations in the line profiles with viewing angles, as diagnosed by the first three moments of the line profiles; shifts, widths, and skewness. We compare line profiles from different elements, and study the morphology of line-of-sight slices that determine the flux at each part of a line profile. We find that excitation conditions can sometimes make the momentum vector of the ejecta emitting in the excited states significantly different from that of the bulk of the ejecta of the respective element, thus giving blueshifted lines for bulk receding material, and vice versa. We compare the 3.3 M-circle dot He-core model to observations of the Type Ib supernova SN 2007Y.