Resolving the fastest ejecta from binary Neutron Star mergers: implications for electromagnetic counterparts

TL;DR

This study investigates how spatial resolution affects the amount of high-velocity ejecta in binary neutron star merger simulations, finding convergence at high resolutions and implications for electromagnetic counterpart detectability.

Contribution

The paper demonstrates that high-resolution grid-based simulations produce converged estimates of fast ejecta, clarifying discrepancies with particle-based methods and informing electromagnetic counterpart predictions.

Findings

Fast ejecta production converges at 20m cell size.

Existing grid-based simulations likely underestimate fast ejecta.

Detectable neutron-powered precursors within 200 Mpc.

Abstract

We examine the effect of spatial resolution on initial mass ejection in grid-based hydrodynamic simulations of binary neutron star mergers. The subset of the dynamical ejecta with velocities greater than $\sim 0.6$c can generate an ultraviolet precursor to the kilonova on $\sim$hr timescales and contribute to a years-long non-thermal afterglow. Previous work has found differing amounts of this fast ejecta, by one- to two orders of magnitude, when using particle-based or grid-based hydrodynamic methods. Here we carry out a numerical experiment that models the merger as an axisymmetric collision in a co-rotating frame, accounting for Newtonian self-gravity, inertial forces, and gravitational wave losses. The lower computational cost allows us to reach spatial resolutions as high as $4$m, or $\sim 3\times 10^{-4}$ of the stellar radius. We find that fast ejecta production converges to within $10\%$ for a cell size of $20$m. This suggests that fast ejecta quantities found in existing grid-based merger simulations are unlikely to increase to the level needed to match particle-based results upon further resolution increases. The resulting neutron-powered precursors are in principle detectable out to distances $\lesssim 200$Mpc with upcoming facilities. We also find that head-on collisions at the free-fall speed, relevant for eccentric mergers, yield fast and slow ejecta quantities of order $10^{-2}M_\odot$, with a kilonova signature distinct from that of quasi-circular mergers.