Between tens and hundreds of days after explosion, the material ejected from Type Ia supernovae become optically thin at most wavelengths. At this time, the observed spectra are dominated by emission from forbidden radiative transitions of singly- and doubly-ionised Fe, Co, and Ni. With almost all of the 56Ni having decayed prior to the nebular phase, most of the energy injection is from the decay of 56Co. The decays of 56Co release gamma rays and positrons, which Compton and Coulomb scatter with thermal electrons, resulting in a population of high-energy (~MeV) non-thermal electrons. These non-thermal electrons are especially important for the ionisation balance, as the rate of non-thermal ionisation is generally higher than photoionisation for the most important ions at this time.
The ARTIS code (Sim 2007, Kromer & Sim 2009) is a multi-dimensional radiative transfer code that uses the Monte Carlo method with indivisible energy packets (Lucy 2002) to compute spectra for modern supernova explosion simulations. To extend the range of validity to late times, we have (2016/17) developed a new non-LTE population/ionisation solver that includes non-thermal ionisation, excitation, and heating processes, and a comprehensive new atomic database with forbidden transitions and modern photoionisation cross sections. This allows us, for the first time, to calculate spectra out to one year post-explosion (early studies with ARTIS were typically limited to less than one month). Extensive testing has verified that our code can reproduce previous one-dimensional work (see Figure 1).
Taking advantage of DiRAC HPC resources, we have calculated nebular spectra for a range of explosion scenarios, including both Chandrasekhar- and sub-Chandrasekhar-mass explosions (Nomoto et al. 1984; Iwamoto et al. 1999; Sim et al. 2010). Results substantiate the potential of the [Ni II] 7378 feature to distinguish between explosion scenarios (Shingles et al.).