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

Abstract

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 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 (SPH) 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 ∼2 brighter than along equatorial lines of sight. Instead of a distinct emission peak, our bolometric light curve exhibits a monotonic decline, characterised 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 day the emission is strongly enhanced and dominated by the secular ejecta, leading to the shift of the shoulder from ∼1-2 to 6-10 days.

Self-consistent 3D Radiative Transfer for Kilonovae: Directional Spectra from Merger Simulations

“…Since the SPH simulation terminates at 20 ms after the merger, we only capture the early, dynamical ejecta, which in our case has a mass of 0.005 M ☉ and is made up of ∼2000 SPH particles. The same NSM simulation was used in 3D radiative transfer calculations with wavelength-independent (gray) opacities by Collins et al (2023).…”

“…For performing 3D frequency-dependent radiative transfer calculations with ARTIS 7 (Sim 2007;Kromer & Sim 2009;Shingles et al 2020), we require a snapshot of the density, composition, and thermal energy of each cell in a Cartesian velocity grid. We use the same SPH mapping technique as Collins et al (2023), in which particles are propagated with constant velocity for 0.5 s after the SPH simulation terminates, except that our grid size is 50 3 (instead of 128 3 ). The SPH densities are then mapped onto the Cartesian velocity grid spanning −0.48c to 0.48c along each axis, from which homologous expansion is assumed (i.e., ρ ∝ t −3 ; see Neuweiler et al 2023 for a study on the homologous assumption for dynamical NSM ejecta).…”

“…With recent claims of observational constraints on the ejecta geometry (e.g., , it is especially important to understand whether multidimensional simulations can be reconciled with observations. While there are studies that consider NSM ejecta distributions directly from simulations in two dimensions (Kasen et al 2015;Kawaguchi et al 2018Kawaguchi et al , 2021Kawaguchi et al , 2022Korobkin et al 2021;Just et al 2022;Bulla 2023) or three dimensions (Tanaka & Hotokezaka 2013;Collins et al 2023;Neuweiler et al 2023), these calculations use some form of simplified opacity treatment, generally by combining line transitions into wavelength bins rather than calculating individual line-by-line opacities.…”

End-to-end Kilonova Models of Neutron Star Mergers with Delayed Black Hole Formation

“…Given the substantial anisotropy of the ejecta (e.g., panels (a) and (b) of Figure 4), we expect that our models show a strong viewing-angle dependence of the KN, e.g., in contrast to models based only on the dynamical ejecta (e.g., Just et al 2022;Collins et al 2023). Since AT 2017gfo was viewed from a polar angle, θ, close to the rotation axis (e.g., Mooley et al 2022), we plot the KN observables averaged over θ < π/4 only (dashed lines in panels (c)-(h)).…”

“…Network A (used previously in, e.g., Goriely et al 2011;Just et al 2015a;Kullmann et al 2023) takes nuclear ingredients from experiments where available and, where not, from theoretical models, namely, nuclear masses from the BSkG2 mass model (Ryssens et al 2022); β-decay rates from Marketin et al (2016); reaction rates from TALYS estimates with microscopic inputs (Goriely et al 2018), including BSkG2 masses; and fission probabilities and fragment distributions from Lemaître et al (2021). Network B (employed previously in, e.g., Wu et al 2016;Collins et al 2023) uses the reaction rates for neutron captures, photodissociation, and fission based on the HFB21 mass model (Goriely et al 2010) as described in Mendoza-Temis et al (2015) and β-decay rates from Marketin et al (2016).…”