3D Binary Neutron Star Merger Ejecta Evolution up to Seconds Timescale: Dynamics, Element Distribution, and Light Curves

This study presents long-term, three-dimensional simulations of ejecta from four binary neutron star mergers, evolved to second-long timescales. The simulations use numerical-relativity data as boundary conditions for a general-relativistic hydrodynamics evolution, incorporating an equation of state valid outside nuclear statistical equilibrium and an effective nuclear-heating prescription derived from reaction-network calculations. The authors investigate the ejecta's dynamical and geometrical properties, the impact of nuclear heating, the formation and spatial distribution of elements, and compute multi-angle kilonova light curves.

Nuclear heating is found to significantly affect ejecta dynamics, delaying homologous expansion beyond second timescales and reshaping the spatial distribution of heavy nuclei. The effect is largest for asymmetric binaries with long-lived remnants; extending the evolution from ~150 ms to ~1 s widens the angular polar region containing 90% of the heavy-element mass (e.g., Z=56, Z=79) from |θ|≲15° to |θ|≲30°. Nucleosynthesis results confirm that the 56Ni→56Co→56Fe decay chain dominates the heating at ~100 days, with cobalt decay producing gamma-ray lines at 846.77 and 1238.288 keV.

Comparing kilonova ray-by-ray light curves obtained from multi-angle 3D profiles and averaged 2D profiles, the authors find the latter approach broadly robust, although 2D light curves should be treated as upper limits. Increasing dimensionality generally lowers the bolometric luminosity, and binary asymmetry strengthens the viewing-angle dependence. For observers aligned with a lanthanide curtain's densest region, 3D emission can match its 2D counterpart in brightness. The authors conclude that increased dimensionality alone is unlikely to reconcile current theoretical models with AT2017gfo observations.