Time-Dependent Radioactive Decay Particle Degradation in Kilonovae

Sammanfattning

Light curve and spectral modelling have been central to advancing our understanding of supernovae and kilonovae. However, despite progress in modelling radioactive energy deposition and radiative transfer, the validity of steady-state approximations during late nebular evolution remains insufficiently explored. This project aims to unify time-dependent electron degradation and channel-specific energy deposition within a single self-consistent framework. In doing so, I develop a time-dependent solver for radioactive decay and particle energy degradation based on the Spencer–Fano equation, including ionisation and heat loss. I use this to investigate the resulting thermalisation efficiencies across a range of ejecta densities and ionisation states and compare them to the corresponding time-dependent degradation spectra and timescales at injection energies of 50 keV and 1 MeV. The degradation time-delays become visible in both the degradation spectrum and the thermalisation efficiency as the ionisation or heat loss timescales become comparable to or exceed the expansion timescale. The calculations reveal the time-dependent evolution of the partitioning of deposited radioactive energy between heating and ionisation channels, a quantity that has not been directly accessible in previous approaches. It was found that the energy bin width has to be sufficiently small to resolve the steep differential cross-section accurately, which otherwise led to the efficiency not reaching unity at 1 MeV injection energy. The code could be extended by including excitation processes for a more complete description of the electron degradation and a continuous ionisation loss treatment for the highest-energy electrons. Further developments could also include implementing the time-dependent solver into SUMO to enable calculations of time-dependent spectra.