Global simulations of strongly magnetized remnant massive neutron stars formed in binary neutron star mergers

We perform a general-relativistic magnetohydrodynamics simulation for $\approx 30$ ms after merger of a binary neutron star to a remnant massive neutron star (RMNS) with a high spatial resolution of the finest grid resolution $12.5$ m. First, we estimate that the Kelvin-Helmholtz instability at merger could amplify the magnetic-field energy up to $\sim 1\mathrm{\%}$ of the thermal energy. Second, we find that the magnetorotational instability in the RMNS envelope and torus with $\rho <{10}^{13}\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$ sustains magneto-turbulent state and the effective viscous parameter in these regions is likely to converge to $\approx 0.01$–$0.02$ with respect to the grid resolution. Third, the current grid resolution is not still fine enough to sustain magneto-turbulent state in the RMNS with $\rho \ge 10^{13}~{\rm g~cm^{-3}}$.