Ab-Initio General-Relativistic Neutrino-Radiation Hydrodynamics Simulations of Long-Lived Neutron Star Merger Remnants to Neutrino Cooling Timescales

TL;DR

This paper delivers the first ab-initio, 3D general-relativistic neutrino-radiation hydrodynamics simulations of a long-lived neutron-star merger remnant extending beyond 100 ms post-merger. It shows neutrino cooling overtaking gravitational-wave energy losses around 20 ms, with a dense $\bar{\nu}_e$-rich outer core and a massive disk that stores angular momentum, allowing the remnant to reach a quasi-steady, stably stratified state while remaining differentially rotating. The results reveal robust neutrino transport effects and show the remnant is stable against convection and the magnetorotational instability, suggesting that other MHD mechanisms must act on longer timescales to remove differential rotation. These findings indicate the RMNS evolves differently from protoneutron stars and have implications for the electromagnetic counterparts of neutron-star mergers, including SGRBs and kilonovae.

Abstract

We perform the first 3D ab-initio general-relativistic neutrino-radiation hydrodynamics of a long-lived neutron star merger remnant spanning a fraction of its cooling time scale. We find that neutrino cooling becomes the dominant energy loss mechanism after the gravitational-wave dominated phase (${\sim}20\ {\rm ms}$ postmerger). Electron flavor anti-neutrino luminosity dominates over electron flavor neutrino luminosity at early times, resulting in a secular increase of the electron fraction in the outer layers of the remnant. However, the two luminosities become comparable ${\sim}20{-}40\ {\rm ms}$ postmerger. A dense gas of electron anti-neutrinos is formed in the outer core of the remnant at densities ${\sim}10^{14.5}\ {\rm g}\ {\rm cm}^{-3}$, corresponding to temperature hot spots. The neutrinos account for ${\sim}10\%$ of the lepton number in this region. Despite the negative radial temperature gradient, the radial entropy gradient remains positive and the remnant is stably stratified according to the Ledoux criterion for convection. A massive accretion disk is formed from the material squeezed out of the collisional interface between the stars. The disk carries a large fraction of the angular momentum of the system, allowing the remnant massive neutron star to settle to a quasi-steady equilibrium within the region of possible stable rigidly rotating configurations. The remnant is differentially rotating, but it is stable against the magnetorotational instability. Other MHD mechanisms operating on longer timescales are likely responsible for the removal of the differential rotation. Our results indicate the remnant massive neutron star is thus qualitatively different from a protoneutron stars formed in core-collapse supernovae.

Figures (10)

• Figure 1: Gravitational wave and neutrino cooling timescales. The data is smoothed using a running average with window width $\Delta t = 0.5 {\rm ms}$. Neutrino radiation becomes the dominant mechanism for the evolution of the remnant ${\sim}10\ {\rm ms}$ after merger.
• Figure 2: GW luminosity and neutrino luminosity and average energies. The data is smoothed using a running average with window width $\Delta t = 0.5\ {\rm ms}$. The GW luminosity is shown for the three most dominant modes: $(\ell,m)=(2,1), (2,2)$, and $(3,3)$. $L_{\nu_\mu}$ is the luminosity of one of the heavy-lepton neutrino species. We do not track separately $\nu_\mu$, $\nu_\tau$ and their antiparticles, so the overall neutrino luminosity of the four species combined is $4\times L_{\nu_\mu}$.
• Figure 3: Angularly averaged profiles of entropy per baryon $s$, electron fraction $Y_e$, convection instability criterion $\chi$, neutrino fractions $Y_{\nu_e}$, $Y_{\bar{\nu}_e}$, and $Y_{\nu_\mu}$, and angular frequency $\Omega$ on the equatorial plane for the $\ell_{\rm mix} = 0$ SR binary. The profiles are shown as a function of cylindrical radius $\varrho$ and time from merger $t - t_{\rm mrg}$. The purple contour denotes $\rho = 10^{13}\ {\rm g}\cdot {\rm cm}^{-3}$, while the green contours are $\rho = 10^{13.5}, 10^{14}$, and $10^{14.5}\ {\rm g}\cdot {\rm cm}^{-3}$.
• Figure 4: Baryonic mass and angular momentum at the end of the simulations for the SR binary. The grey shaded area shows the set of all rigidly-rotating equilibrium configurations. The green line shows the evolution of the angular momentum due to gravitational wave emission, while the red and blue line show the evolution of the baryonic mass and angular momentum for the disk and the remnant, respectively. The remnant settles to a region corresponding to a stable rigidly-rotating configuration, but it is still rotating differentially at the end of our simulations
• Figure 5: Mixing length prescription used for the viscous simulations. The data points are obtained by combining the values of $\alpha$ reported by Kiuchi:2017zzgKiuchi:2022nin with angular velocities and sound speed values of Radice:2020ids. The solid line is the fitting function adopted here.