A connection between atomic physics and gravitational wave spectroscopy

Neutron star mergers are promising candidates for the observation of an electromagnetic signal coincident with gravitational waves. The properties of the ejecta produced during these events are expected to play an important role in the electromagnetic transients called macronovae. Characteristics of the ejecta include large velocity gradients and the presence of heavy r-process elements, which pose significant challenges to the accurate calculation of radiative opacities and radiation transport. For example, these opacities include a dense forest of bound-bound features arising from near-neutral lanthanide and actinide elements. Here we investigate the use of fine-structure, line-broadened opacities that take into account the motional Doppler broadening caused by the velocity gradients. The use of individual line profiles produces frequency-dependent opacities that are one to two orders of magnitude greater than those obtained with the commonly used expansion opacities in the Sobolev approximation. These lower opacities result in simulated emission from neutron star mergers that is significantly dimmer and shifted toward the infra-red spectrum than previously predicted.