Low-Temperature Lanthanide Spectroscopy Applied to Neutron Star Mergers

Neutron star mergers are promising candidates for the observation of an electromagnetic (EM) signal coincident with gravitational waves. The recent observation of GW170817 [1] appears to be such an event, with gravitational waves confirmed by subsequent EM signals ranging from the infrared to x-ray portions of the spectrum. This EM emission is powered by the radioactive decay of heavy r-process elements, and is called a kilonova. The atomic properties, i.e. radiative opacity, of these elements in the resulting ejecta play an important role in determining the characteristics of the radiation that ultimately reaches the Earth. For example, the opacities of r-process elements, such as the lanthanides, include a dense forest of bound-bound features that result in the strong absorption of radiation over a significant range of photon energies. In this work, we use the Los Alamos suite of atomic physics and plasma modeling codes [2] to investigate the use of detailed, fine-structure opacities [3,4] to model the EM emission from kilonovae. Our simulations, e.g. [5], predict emission in a range of EM bands, depending on issues such as the presence of winds, elemental composition, and viewing angle.