Free Neutron Ejection From Shock Breakout in Binary Neutron Star Mergers

TL;DR

This study uses relativistic hydrodynamics simulations to estimate free neutron ejection from neutron star mergers, finding significantly less neutrons than previously thought and suggesting potential optical emissions post-merger.

Contribution

It provides the first systematic estimate of free neutron mass ejected during neutron star mergers using simplified shock models.

Findings

Free neutron mass ejected is $10^{-7}$ to $10^{-6}\,M_{ m ext{sun}}$

Maximum free neutrons occur at shock energy around $10^{48}$ erg

Optical luminosity estimated at $7 imes 10^{41}$ erg/s about 30 minutes post-merger

Abstract

Merging neutron stars generate shock waves that disintegrate heavy nuclei into nucleons especially in the outer envelope. It is expected that some of such neutrons having avoided capturing positrons remain as free neutrons even after disappearance of electron-positron pairs. To investigate how many free neutrons can be ejected from merging neutron stars, we performed special-relativistic Lagrangian hydrodynamics computation with simplified models of this phenomenon in which a spherically symmetric shock wave propagates in the hydrostatic envelope and emerge from the surface. We systematically study a wide parameter space of the size of the merging neutron stars and the energy involved in the shock waves. As a result, it is found that the mass of remaining free neutron is $10^{-7}$ to $10^{-6}\ M_{\odot}$, which is smaller than the previously expected by more than two orders of magnitude. There is a preferred energy of the order of $10^{48}$ erg that yields the maximum amount of free neutrons for large size of the envelope. We briefly discuss the emission from the free neutron layer and estimate the luminosity in the optical band to be about $7\times 10^{41}$ erg s $^{-1}(M_{\rm n}/10^{-6}\ M_\odot)$ in $\sim 30$ min after the merger.