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Merging neutron stars generate shock waves that disintegrate heavy nucleiinto nucleons especially in the outer envelope. It is expected that some ofsuch neutrons having avoided capturing positrons remain as free neutrons evenafter disappearance of electron-positron pairs. To investigate how many freeneutrons can be ejected from merging neutron stars, we performedspecial-relativistic Lagrangian hydrodynamics computation with simplifiedmodels of this phenomenon in which a spherically symmetric shock wavepropagates in the hydrostatic envelope and emerge from the surface. Wesystematically study a wide parameter space of the size of the merging neutronstars and the energy involved in the shock waves. As a result, it is found thatthe mass of remaining free neutron is $10^{-7}$ to $10^{-6}\ M_{\odot}$, whichis 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 themaximum amount of free neutrons for large size of the envelope. We brieflydiscuss the emission from the free neutron layer and estimate the luminosity inthe 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.

Free Neutron Ejection From Shock Breakout in Binary Neutron Star Mergers