We perform a general-relativistic magnetohydrodynamics simulation for $\approx 30\mathrm{ms}$ from the merger of a binary neutron star throughout the formation of a remnant massive neutron star (RMNS) with a high spatial resolution of the finest grid resolution 12.5 m. First, we show that the Kelvin-Helmholtz instability at the merger could amplify the magnetic-field energy at least up to $\sim 1\%$ of the thermal energy. Then, we show that the magnetorotational instability in the RMNS envelope and torus with $\rho <{10}^{13}\mathrm{g}{\mathrm{cm}}^{-3}$ sustains the magnetoturbulent 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. We also point out that the current grid resolution is not still fine enough to sustain a magnetoturbulent state in the RMNS with $\rho \ge {10}^{13}\mathrm{g}{\mathrm{cm}}^{-3}$.

• Received 5 October 2017

DOI:https://doi.org/10.1103/PhysRevD.97.124039