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

TL;DR

This study models kilonova emission from neutron star merger ejecta using advanced simulations and radiation transport, revealing detailed light curve features and emphasizing the importance of neutrino physics, but finds models are dimmer than observed events.

Contribution

First to analyze dynamical ejecta from neutron star mergers with detailed neutrino and nucleosynthesis modeling using multi-dimensional radiation transport.

Findings

Kilonova peaks after 0.7-1.5 days in near-infrared.

Emission is 1.5-3 times stronger towards the pole.

Models do not reproduce the brightness of AT2017gfo.

Abstract

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 ~10ms 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 multi-dimensional 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 0.7-1.5days in the near-infrared frequency regime with luminosities between 3-7x10^40erg/s and at photospheric temperatures of 2.2-2.8x10^3K. 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 modeling. 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.