The impact of r-process heating on the dynamics of neutron star merger accretion disc winds and their electromagnetic radiation

Abstract

Neutron star merger accretion discs can launch neutron-rich winds of >10−2M⊙. This ejecta is a prime site for r-process nucleosynthesis, which will produce a range of radioactive heavy nuclei. The decay of these nuclei releases enough energy to accelerate portions of the wind by ∼0.1c. Here, we investigate the effect of r-process heating on the dynamical evolution of disc winds. We extract the wind from a 3D general relativistic magnetohydrodynamic simulation of a disc from a post-merger system. This is used to create inner boundary conditions for 2D hydrodynamic simulations that continue the original 3D simulation. We perform two such simulations: one that includes the r-process heating, and another one that does not. We follow the hydrodynamic simulations until the winds reach homology (60 seconds). Using time-dependent multi-frequency multi-dimensional Monte Carlo radiation transport simulations, we then calculate the kilonova light curves from the winds with and without dynamical r-process heating. We find that the r-process heating can substantially alter the velocity distribution of the wind, shifting the mass-weighted median velocity from 0.06c to 0.12c. The inclusion of the dynamical r-process heating makes the light curve brighter and bluer at ∼1 d post-merger. However, the high-velocity tail of the ejecta distribution and the early (≲ 1 d) light curves are largely unaffected.

Late-time post-merger modeling of a compact binary: effects of relativity, r-process heating, and treatment of transport effects

“…However, a reliable model of these effects will require a consistent treatment of neutrino losses. Another expected effect of r-process heating on ejecta is a smoothing of spatial inhomogeneities, but this would not be visible on the length and timescales of this simulation [44,22,45]. In Figure 7, we investigate the sensitivity of our distributions to the observation radius and time.…”

Long-term 3D-MHD Simulations of Black Hole Accretion Disks formed in Neutron Star Mergers

“…Nuclear recombination deposits additional energy into the already unbound outflows at t ∼ 0.1 s, resulting in ejecta with moderately higher velocities, but its absence only decreases the total ejecta mass by 1%. Inclusion of r-process heating by the formation of heavier nuclei can further speed up the ejecta at late times (Klion et al 2022). Proper characterization of the effect of neutrino absorption and nuclear recombination on the mass ejection dynamics and composition must be done with full 3D simulations, which unfortunately still remain expensive computationally.…”

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.…”

“…We note that these mappings between electron fraction and opacity are typically independent of the temperature or calibrated to the temperature when the light curve peaks (Tanaka et al 2020) and may not be appropriate for the first few hours when magnetar spindown energy is most relevant. Other features such as shock-heating, jet-interaction or viewing angle dependence may also muddy the waters (Klion et al 2021;Nativi et al 2021;Klion et al 2022) suggesting that the colour evolution alone may be an inconclusive diagnostic.…”