Jets from neutron-star merger remnants and massive blue kilonovae

TL;DR

This study uses advanced simulations to show that neutron star mergers produce jets and ejecta that can explain observed kilonova signals and early blue emission in events like GW170817.

Contribution

It provides the first self-consistent simulation of jet formation and ejecta from long-lived neutron star merger remnants, matching observed kilonova features.

Findings

Jets emerge within 30 ms post-merger with Lorentz factors 5-10.

Ejecta of over 0.02 solar masses are launched within 50 ms, consistent with GW170817.

A kilonova precursor due to a magnetized wind is produced hours after merger.

Abstract

We perform high-resolution three-dimensional general-relativistic magnetohydrodynamic simulations with neutrino transport of binary neutron star (BNS) mergers resulting in a long-lived remnant neutron star, with properties typical of galactic BNS and consistent with those inferred for the first observed BNS merger GW170817. We demonstrate self-consistently that within $\lesssim\!30$ ms post-merger magnetized ($\sigma\sim 5-10$) twin polar jets emerge with asymptotic Lorentz factor $\Gamma\sim 5-10$, which successfully break out from the merger debris within $\lesssim\!20$ ms. A fast ($v\lesssim 0.6c$), magnetized ($\sigma\sim 0.1$) wind surrounds the jet core and generates a UV/blue kilonova precursor on timescales of hours, similar to the precursor signal due to free neutron decay in fast dynamical ejecta. Post-merger ejecta are quickly dominated by MHD-driven outflows from an accretion disk. We demonstrate that within only 50 ms post-merger, $\gtrsim 2\times 10^{-2}M_\odot$ of lanthanide-free, quasi-spherical ejecta with velocity $\sim\!0.1c$ is launched, yielding a kilonova signal consistent with GW170817 on timescales of $\lesssim\!5$\,d.