# Spectra of supernovae in the nebular phase

## TL;DR

This paper reviews the spectral formation processes of supernovae during the nebular phase, linking spectral features to physical conditions, explosion mechanisms, and ejecta morphology, with derivations and applications of models.

## Contribution

It provides a comprehensive review of the physical processes, formalism, and modeling techniques for nebular-phase supernova spectra, including new derivations and applications.

## Key findings

- Line profiles encode ejecta morphology.
- Luminosity expressions derived for different physical limits.
- Physical processes like gamma-ray deposition and ionization are linked to spectral models.

## Abstract

When supernovae enter the nebular phase after a few months, they reveal spectral fingerprints of their deep interiors, glowing by radioactivity produced in the explosion. We are given a unique opportunity to see what an exploded star looks like inside. The line profiles and luminosities encode information about physical conditions, explosive and hydrostatic nucleosynthesis, and ejecta morphology, which link to the progenitor properties and the explosion mechanism. Here, the fundamental properties of spectral formation of supernovae in the nebular phase are reviewed. The formalism between ejecta morphology and line profile shapes is derived, including effects of scattering and absorption. Line luminosity expressions are derived in various physical limits, with examples of applications from the literature. The physical processes at work in the supernova ejecta, including gamma-ray deposition, non-thermal electron degradation, ionization and excitation, and radiative transfer are described and linked to the computation and application of advanced spectral models. Some of the results derived so far from nebular-phase supernova analysis are discussed.