Opacity of the highly ionized lanthanides and the effect on the early kilonova

TL;DR

This study examines how highly ionized lanthanides affect early kilonova light curves, revealing increased opacities that cause the emission to fade faster and predicting observable UV signatures within hours after neutron star mergers.

Contribution

It provides the first detailed calculations of atomic structures and opacities for highly ionized lanthanides, demonstrating their significant impact on early kilonova emission.

Findings

Highly ionized lanthanides have exceptionally high opacities (~1000 cm^2/g).

Early kilonova light curves are fainter by up to a factor of four due to lanthanide opacity.

UV brightness peaks around 0.2 days at magnitude ~19 for lanthanide-rich ejecta.

Abstract

We investigate the effect of the presence of lanthanides (Z = 57- 71) on the kilonova at t~hours after the neutron star merger for the first time. For this purpose, we calculate the atomic structures and the opacities for selected lanthanides: Nd (Z = 60), Sm (Z = 62), and Eu (Z = 63). We consider the ionization degree up to tenth (XI), applicable for the ejecta at t ~ a few hours after the merger, when the temperature is T ~ 10^5 K. We find that the opacities for the highly ionized lanthanides are exceptionally high, reaching k_exp~1000 cm^2/g for Eu, due to the highly dense energy levels. Using the new opacity, we perform radiative transfer simulations to show that the early light curves become fainter by a (maximum) factor of four, in comparison to lanthanide-free ejecta at t~0.1 day. However, the period at which the light curves are affected is relatively brief due to the rapid time evolution of the opacity in the outermost layer of the ejecta. We predict that for a source at a distance of ~100 Mpc, UV brightness for lanthanide-rich ejecta shows a drop to ~21-22 mag at t~0.1 day and the UV peaks around t~0.2 day with a magnitude of ~19 mag. Future detection of such a kilonova by the existing UV satellite like Swift or the upcoming UV satellite ULTRASAT will provide useful constraints on the abundance in the outer ejecta and the corresponding nucleosynthesis conditions in the neutron star mergers.