The role of viscosity in binary neutron star mergers

For the first time, researchers self-consistenly model a binary neutron star merger and its long-term evolution with viscous hydrodynamics and its ejecta

June 12, 2018

Using data representing a remnant of a binary neutron star merger from a numerical relativity simulation, this study evolves the resulting massive neutron star (MNS) in an axisymmetric long-term general relativistic neutrino radiation hydrodynamics simulation with viscosity. The viscosity plays a key role for the mass ejection and induces new effects: 1) when the MNS switches from differential to rigid rotation and 2) in the evolution of the torus surrounding the MNS. The results predict that the electromagnetic emission from the viscosity-driven ejecta could reproduce the optical and infrared observations associated with GW170817.

Paper abstract

Snapshots of the density and poloidal velocity field for the fiducial model DD2-135135-0.02-H at t = 2.4 s. The length of the velocity vector corresponds to the logarithm of the poloidal velocity. The left and right subpanels show a wide region and narrow region, respectively.

We perform long-term general relativistic neutrino radiation hydrodynamics simulations (in axisymmetry) for a massive neutron star (MNS) surrounded by a torus, which is a canonical remnant formed after the binary neutron star merger. We take into account the effects of viscosity, which is likely to arise in the merger remnant due to magnetohydrodynamical turbulence. The viscous effect plays key roles for the mass ejection from the remnant in two phases of the evolution. In the first t≲10 ms, a differential rotation state of the MNS is changed to a rigidly rotating state. A shock wave caused by the variation of its quasi-equilibrium state induces significant mass ejection of mass ∼(0.5–2.0)x10-2 M⊙︎ for the α-viscosity parameter of 0.01–0.04. For the longer-term evolution with ∼0.1–10 s, a significant fraction of the torus material is ejected. We find that the total mass of the viscosity-driven ejecta (≳10-2 M⊙︎) could dominate over that of the dynamical ejecta (≲10-2 M⊙︎). The electron fraction, Ye, of the ejecta is always high enough (Ye≳0.25) that this post-merger ejecta is lanthanide-poor; hence, the opacity of the ejecta is likely to be ∼10–100 times lower than that of the dynamical ejecta. This indicates that the electromagnetic signal from the ejecta would be rapidly evolving, bright, and blue if it is observed from a small viewing angle (≲45°) for which the effect of the dynamical ejecta is minor.

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