NLTE Effects on Kilonova Expansion Opacities

TL;DR

This paper investigates the impact of non-local thermodynamic equilibrium (NLTE) effects on kilonova expansion opacities, revealing that NLTE modeling is crucial for accurate late-phase predictions of kilonova spectra.

Contribution

First analysis of NLTE level populations in kilonova ejecta using spectral synthesis code SUMO, comparing to LTE, and highlighting the importance of NLTE effects over time.

Findings

NLTE effects cause deviations in level populations a few days after merger.

NLTE leads to lower expansion opacities for most ions and states.

Opacity differences grow to factors of 2-10 after a few days.

Abstract

A binary neutron star merger produces a rapidly evolving transient known as a kilonova (KN), which peaks a few days after merger. Modelling of KNe has often been approached assuming local thermodynamic equilibrium (LTE) conditions in the ejecta. We present the first analysis of non-local thermodynamic equilibrium (NLTE) level populations, using the spectral synthesis code SUMO, and compare these to LTE values. We investigate the importance of the radiation field by conducting NLTE excitation calculations with and without radiative transfer. Level populations, in particular higher lying ones, start to show deviations from LTE a few days after merger. Excitation is lower in NLTE for the majority of ions and states, and this tends to give lower expansion opacities. While the difference is small for the first few days, it grows to factors 2-10 after this. Our results are important both for demonstrating validity of LTE expansion opacities for an initial phase (few days), while highlighting the need for NLTE modelling during later phases. Considering also NLTE ionisation, our results indicate that NLTE can give both higher or lower opacities, depending on composition and wavelength, sometimes by orders of magnitudes.