3D radiative transfer kilonova modelling for binary neutron star merger simulations

The detection of GW170817 and the accompanying electromagnetic counterpart, AT2017gfo, have provided an important set of observational constraints for theoretical models of neutron star mergers, nucleosynthesis, and radiative transfer for kilonovae. We apply the three-dimensional (3D) Monte Carlo radiative transfer code artis to produce synthetic light curves of the dynamical ejecta from a neutron star merger, which has been modelled with 3D smooth particle hydrodynamics and included neutrino interactions. Nucleosynthesis calculations provide the energy released from radioactive decays of r-process nuclei, and radiation transport is performed using grey opacities given as functions of the electron fraction. We present line-of-sight-dependent bolometric light curves, and find the emission along polar lines of sight to be up to a factor of similar to 2 brighter than that along equatorial lines of sight. Instead of a distinct emission peak, our bolometric light curve exhibits a monotonic decline, characterized by a shoulder at the time when the bulk ejecta becomes optically thin. We show approximate band light curves based on radiation temperatures and compare these to the observations of AT2017gfo. We find that the rapidly declining temperatures lead to a blue to red colour evolution similar to that shown by AT2017gfo. We also investigate the impact of an additional, spherically symmetric secular ejecta component, and we find that the early light curve remains nearly unaffected, while after about 1 d the emission is strongly enhanced and dominated by the secular ejecta, leading to the shift of the shoulder from similar to 1-2 to 6-10 d.

Automated summary

The detection of GW170817 and its electromagnetic counterpart AT2017gfo has provided valuable constraints for theoretical models of neutron star mergers and nucleosynthesis. Using a Monte Carlo radiative transfer code, the authors simulate synthetic light curves of the dynamical ejecta from a neutron star merger, taking into account neutrino interactions. They find that emission along polar lines of sight is brighter than that along equatorial lines of sight, and the bolometric light curve exhibits a monotonic decline with a shoulder when the ejecta becomes optically thin. The observed color evolution of AT2017gfo is reproduced by the simulation. The addition of a spherically symmetric secular ejecta component enhances the emission, leading to a shift in the shoulder time from 1-2 days to 6-10 days.