Radiation hydrodynamics modelling of kilonovae withSNEC

Abstract

We develop a method to compute synthetic kilonova light curves that combines numerical relativity simulations of neutron star mergers and the SNEC radiation-hydrodynamics code. We describe our implementation of initial and boundary conditions, r-process heating, and opacities for kilonova simulations. We validate our approach by carefully checking that energy conservation is satisfied and by comparing the SNEC results with those of two semi-analytic light curve models. We apply our code to the calculation of colour light curves for three binaries having different mass ratios (equal and unequal mass) and different merger outcome (short-lived and long-lived remnants). We study the sensitivity of our results to hydrodynamic effects, nuclear physics uncertainties in the heating rates, and duration of the merger simulations. We find that hydrodynamics effects are typically negligible and that homologous expansion is a good approximation in most cases. However, pressure forces can amplify the impact of uncertainties in the radioactive heating rates. We also study the impact of shocks possibly launched into the outflows by a relativistic jet. None of our models match AT2017gfo, the kilonova in GW170817. This points to possible deficiencies in our merger simulations and kilonova models which neglect non-LTE effects and possible additional energy injection from the merger remnant, and to the need to go beyond the assumption of spherical symmetry adopted in this work.

Electromagnetic Counterparts of Binary-neutron-star Mergers Leading to a Strongly Magnetized Long-lived Remnant Neutron Star

“…This hydrodynamical effect in the subsequent evolution could be reflected in the light curve of the EM counterparts. Currently, there are only a limited number of studies that consider the long-term hydrodynamics evolution of ejecta to predict the property of the EM counterparts (Rosswog et al 2014;Grossman et al 2014;Fernández et al 2015Fernández et al , 2017Foucart et al 2021;Kawaguchi et al 2021;Wu et al 2022).…”

On the diversity of magnetar-driven kilonovae

“…Another source of uncertainty is nuclear physics itself, which has been demonstrated previously to contribute to up to an order of magnitude uncertainty at peak (Zhu et al 2021;Barnes et al 2021). Both these factors, alongside other issues explored by numerical simulations (e.g., Kawaguchi et al 2022;Wu et al 2022), the sig-nificance of neutron precursor emission (Metzger et al 2015a), shock heated ejecta (Gottlieb et al 2018;Piro & Kollmeier 2018), neutrino-driven winds (Metzger et al 2018), interaction with the jet (Klion et al 2021;Nativi et al 2021Nativi et al , 2022, and viewing angle dependencies (Klion et al 2022) indicate systematic uncertainties, which until resolved suggest that the relative brightness of a kilonova alone may not a good diagnostic for distinguishing an engine-driven kilonova from an ordinary one. This is especially true for "Case 2"like systems unless they are observed quite early.…”

On the diversity of magnetar-driven kilonovae

“…Another source of uncertainty is nuclear physics itself, which has been demonstrated previously to contribute to up to an order of magnitude uncertainty at peak (Zhu et al 2021;Barnes et al 2021). Both these factors, alongside other issues explored by numerical simulations (e.g., Kawaguchi et al 2022;Wu et al 2022), the significance of neutron precursor emission (Metzger et al 2015a), shock heated ejecta (Gottlieb et al 2018;Piro & Kollmeier 2018), neutrino-driven winds (Metzger et al 2018), interaction with the jet (Klion et al 2021;Nativi et al 2021Nativi et al , 2022, and viewing angle dependencies (Klion et al 2022) indicate systematic uncertainties, which until resolved suggest that the relative brightness of a kilonova alone may not a good diagnostic for distinguishing an engine-driven kilonova from an ordinary one, especially for "Case 2"-like systems unless they are observed quite early.…”