Three-dimensional GRMHD simulations of neutrino-cooled accretion disks from neutron star mergers

TL;DR

This paper presents the first 3D GRMHD simulations of neutrino-cooled accretion disks from neutron star mergers, revealing mechanisms for outflows that synthesize heavy elements and match kilonova observations.

Contribution

It introduces comprehensive 3D GRMHD simulations including weak interactions and realistic EOS, showing self-regulation, turbulence, and outflows in merger disks.

Findings

Steady-state MHD turbulence and magnetic dynamo observed.

Outflows unbind 40% of disk mass with velocities ~0.1c.

Simulations produce r-process element synthesis consistent with solar abundances.

Abstract

Merging binaries consisting of two neutron stars (NSs) or an NS and a stellar-mass black hole typically form a massive accretion torus around the remnant black hole or long-lived NS. Outflows from these neutrino-cooled accretion disks represent an important site for $r$-process nucleosynthesis and the generation of kilonovae. We present the first three-dimensional, general-relativistic magnetohydrodynamic (GRMHD) simulations including weak interactions and a realistic equation of state of such accretion disks over viscous timescales ($380\,\mathrm{ms}$). We witness the emergence of steady-state MHD turbulence, a magnetic dynamo with an $\sim\!20\,\mathrm{ms}$ cycle, and the generation of a `hot' disk corona that launches powerful thermal outflows aided by the energy released as free nucleons recombine into $\alpha$-particles. We identify a self-regulation mechanism that keeps the midplane electron fraction low ($Y_e\sim0.1$) over viscous timescales. This neutron-rich reservoir, in turn, feeds outflows that retain a sufficiently low value of $Y_e\approx 0.2$ to robustly synthesize third-peak $r$-process elements. The quasi-spherical outflows are projected to unbind $40\%$ of the initial disk mass with typical asymptotic escape velocities of $0.1c$, and may thus represent the dominant mass ejection mechanism in NS-NS mergers. Including neutrino absorption, our findings agree with previous hydrodynamical $\alpha-$disk simulations that the entire range of $r$-process nuclei from the first to the third $r$-process peak can be synthesized in the outflows, in good agreement with observed solar system abundances. The asymptotic escape velocities and the quantity of ejecta, when extrapolated to moderately higher disk masses, are consistent with those needed to explain the red kilonova emission following the NS merger GW170817.