Modelling Kilonova Spectra with NLTE Physics

by Mr Quentin Perceval Xavier Pognan (Stockholm University)

Friday 8 Dec 2023, 10:30 → 11:30 Europe/Stockholm

LC61 (AlbaNova Main Building)

The rapid neutron capture (r-process) has long been theorised to produce approximately half of the elements heavier than iron in the Universe. A promising site of r-process nucleosynthesis is believed to be neutron star (NS) mergers. In 2017, the first ever binary NS merger was observed thoroughly in gravitational wave emission, as well as in a broad-band electromagnetic (EM) follow up. This EM follow up observed the famous kilonova (KN) AT2017gfo, powered by the radioactive decay of unstable r-process isotopes created in the merger ejecta. Since these transients are powered by r-process species, it follows that the analysis of features in KN emission can lead to the identification of elements produced in NS mergers, and place constraints on their origins in the Universe.

A promising approach to do so is by spectral analysis of the emergent KN spectra. However, this requires detailed spectral models which can explain the diverse, complex features in the observed emission. In particular, due to the rapid expansion of merger ejecta moving at velocities of ~ 0.1 - 0.3c, the KN quickly moves away from the photospheric phase, where Local Thermodynamic Equilibrium (LTE) physics apply, into the non-LTE regime. In this regime, the gas state of the ejecta cannot be determined using the Saha-Boltzmann equations, and must instead be found by solving rate equations coupling diverse non-thermal processes, which greatly increases the complexity of the problem. In this talk, I will go over the spectral modelling of KNe in the NLTE regime, firstly introducing the context and difficulties of this approach, and then focussing on recent works and results making use of NLTE radiative transfer codes to this end.

Biographical info: Completed my undergraduate in Physics at McGill University, Canada, in 2016, then MSc. Physics at ETH Zurich, Switzerland in 2018. I've been a PhD at SU since September 2019, and will be defending my thesis on December 18th this year. I hope to continue a career in academia and research.