Non-Local Thermodynamic Equilibrium Spectral Modelling of Kilonovae

The astrophysical origin of rapid neutron capture (r-process) elements has long remained a puzzle and been the object of scientific debate. Neutron star (NS) mergers have historically been suggested as an ideal site for the creation of these elements, and were propelled into focus following the detection of the first binary neutron star (BNS) merger in 2017. The gravitational wave (GW) signal GW170817 was accompanied by a short gamma-ray burst (sGRB) GRB170817A, and a radioactively powered electromagnetic (EM) transient AT2017gfo, known as a kilonova (KN). Since this detection, the study of NS mergers has greatly expanded across the diverse fields that model the various stages of the merger, from GW signal modelling, to radiative transfer studies predicting the emergent KN lightcurves (LCs) and spectra. One main goal of studying NS mergers and the associated KNe is to establish the importance of compact object mergers as key sites of r-process nucleosynthesis in the Universe. As such, identification of elements and their abundances within the merger ejecta represents a critical objective. LC and spectral analyses of KNe provide promising channels to do so, and require detailed models in order to interpret observational data. With complete GW and multi-band EM data only available for a single object thus far, the importance of detailed models regarding every aspect of KN physics is paramount. KN simulations typically make use of radiative transfer (RT) codes that propagate photons through the expanding ejecta, in order to provide LC and spectral outputs. These often model the early, photospheric times of the KN, when the ejecta are still dense enough such that the gas state is well described by Local Thermodynamic Equilibrium (LTE) conditions, which requires thermal collisional processes to dominate within the ejecta. Since the ejecta are expanding rapidly however, these conditions cease to apply after several days, and the KN transitions to the Non-Local Thermodynamic Equilibrium (NLTE) regime, where thermal collisional processes are no longer dominant in establishing the gas state of the ejecta. This now requires the detailed modelling of various NLTE processes which increases the complexity, yet modelling of this regime can also provide great rewards. Notably, as times goes on and the ejecta continue to expand, they will eventually become optically thin to most wavelengths and enter the nebular phase. There, the spectra are expected to be emission line dominated, providing an excellent opportunity for element identification by spectral analysis. This doctoral thesis conducts RT modelling in order to explore the NLTE regime of the KN in a systematic, physically accurate way. To this end, the spectral synthesis code SUMO (SUpernova MOnte Carlo Code) was adapted to model KNe, and used to investigate the spectral emission in the NLTE regime. The work in this doctoral thesis provides a first step into fully consistent modelling and analysis of KNe at later times, and a solid foundation from which to move forwards