Mass Ejection from the Remnant of a Binary Neutron Star Merger: Viscous-radiation Hydrodynamics Study

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 less than or similar to 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 similar to(0.5-2.0) x 10(-2) M circle dot for the alpha-viscosity parameter of 0.01-0.04. For the longer-term evolution with similar to 0.1-10s, a significant fraction of the torus material is ejected. We find that the total mass of the viscosity-driven ejecta (greater than or similar to 10(-2) M circle dot) could dominate over that of the dynamical ejecta (less than or similar to 10(-2) M circle dot). The electron fraction, Y-e, of the ejecta is always high enough (Y-e greater than or similar to 0.25) that this post-merger ejecta is lanthanide-poor; hence, the opacity of the ejecta is likely to be similar to 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 (less than or similar to 45 degrees) for which the effect of the dynamical ejecta is minor.