Dynamical ejecta of neutron star mergers with nucleonic weak processes II: kilonova emission

The majority of existing results for the kilonova (or macronova) emission from material ejected during a neutron-star (NS) merger is based on (quasi-) one-zone models or manually constructed toy-model ejecta configurations. In this study, we present a kilonova analysis of the material ejected during the first similar to 10 ms of a NS merger, called dynamical ejecta, using directly the outflow trajectories from general relativistic smoothed-particle hydrodynamics simulations, including a sophisticated neutrino treatment and the corresponding nucleosynthesis results, which have been presented in Part I of this study. We employ a multidimensional two-moment radiation transport scheme with approximate M1 closure to evolve the photon field and use a heuristic prescription for the opacities found by calibration with atomic-physics-based reference results. We find that the photosphere is generically ellipsoidal but augmented with small-scale structure and produces emission that is about 1.5-3 times stronger towards the pole than the equator. The kilonova typically peaks after 0.7-1.5 d in the near-infrared frequency regime with luminosities between 3-7 x10(40) erg s(-1) and at photospheric temperatures of 2.2-2.8 x 10(3) K. A softer equation of state or higher binary-mass asymmetry leads to a longer and brighter signal. Significant variations of the light curve are also obtained for models with artificially modified electron fractions, emphasizing the importance of a reliable neutrino-transport modelling. None of the models investigated here, which only consider dynamical ejecta, produces a transient as bright as AT2017gfo. The near-infrared peak of our models is incompatible with the early blue component of AT2017gfo.

Automated summary

This study analyzes the kilonova emission from the material ejected during the early phase of a neutron star merger using hydrodynamics simulations. The results show that the photosphere is generally ellipsoidal with small-scale structures, and the emission is stronger towards the pole than the equator. The kilonova peaks in the near-infrared frequency regime after 0.7-1.5 days with varying luminosities and photospheric temperatures. The study also highlights the importance of reliable neutrino transport modeling.