Viscous-Dynamical Ejecta from Binary Neutron Star Merger

TL;DR

This paper presents simulations showing that turbulent viscosity in binary neutron star mergers can produce a new type of fast, massive ejecta with observable electromagnetic signatures, including bright kilonovae and radio rebrightening.

Contribution

It introduces a novel viscous-dynamical ejecta mechanism operating during neutron star mergers, highlighting the role of turbulence-induced viscosity in ejecta properties.

Findings

Ejecta velocities up to 0.8c observed.

Ejecta mass can increase significantly with viscosity.

Potential for bright kilonovae and radio rebrightening signals.

Abstract

General-relativistic simulations of binary neutron star mergers with viscosity reveal a new outflow mechanism operating in unequal mass binaries on dynamical timescales and enabled by turbulent viscosity. These "viscous-dynamical" ejecta are launched during merger due to the thermalization of mass exchange streams between the secondary and the primary neutron star. They are characterized by asymptotic velocities extending up to ${\sim} 0.8\, c$, and have masses that depend on the efficiency of the viscous mechanism. Depending on the unknown strength of the effective viscosity arising from magnetohydrodynamics instabilities operating during merger, the overall mass of the dynamical ejecta could be enhanced by a factor of a few and the mass of the fast tail of the ejecta having asymptotic velocities $\geq 0.6\, c$ by up to four orders of magnitude. The radioactive decay of the expanding viscous-dynamical ejecta could produce bright kilonova transients with signatures of free neutron decay in the first hour and enhanced near infrared flux on a timescale of a few days. The synchrotron remnant produced by the interaction between the ejecta and the interstellar medium could also be significantly enhanced by viscosity. Such remnant could be detected in the case of GW170817 as a rebrightening of the radio signal in the next months to years.