Abstract
Long-term neutrino-radiation resistive-magnetohydrodynamics simulations in full general relativity are performed for a system composed of a massive neutron star and a torus formed as a remnant of binary neutron-star mergers. The simulation is performed in axial symmetry incorporating a mean-field dynamo term for a hypothetical amplification of the magnetic-field strength. We first calibrate the mean-field dynamo parameters by comparing the results for the evolution of black hole--disk systems with viscous hydrodynamics results. We then perform simulations for the system of a remnant massive neutron star and a torus. As in the viscous hydrodynamics case, the mass ejection occurs primarily from the torus surrounding the massive neutron star. The total ejecta mass and electron fraction in the new simulation are similar to those in the viscous hydrodynamics case. However, the velocity of the ejecta can be significantly enhanced by magnetohydrodynamics effects caused by global magnetic fields.
Highlights
The first observation of the binary neutron-star merger GW170817 [1,2] showed that theoretical modeling for the merger and postmerger phases of binary neutron stars is the key for extracting valuable information from the observed electromagnetic signals
We performed general-relativistic neutrino-radiation resistive-MHD (GRRRMHD) simulations incorporating a mean-field dynamo term for black hole–disk systems and for a merger remnant of binary neutron stars composed of a massive neutron star and a torus, paying particular attention to the α − Ω dynamo effect
We compared the new results with those previously obtained in our viscous hydrodynamics simulations [29,30,31] and clarified the specific MHD effects
Summary
The first observation of the binary neutron-star merger GW170817 [1,2] showed that theoretical modeling for the merger and postmerger phases of binary neutron stars is the key for extracting valuable information from the observed electromagnetic signals. In the presence of the resultant strong toroidal field, several magnetohydrodynamics (MHD) instabilities such as the Parker and Taylor instabilities [43,44] together with the convection and circulation can take place, and the magnetic-field strength could be amplified through the dynamo action To accurately investigate this amplification process, we need a high-resolution and long-term MHD simulation in full general relativity with the relevant microphysics such as the neutrino transport that can induce the convection and dynamo [45,46]. These works have illustrated that with the incorporation of the mean-field dynamo term (i.e., with the α − Ω dynamo effect), the numerical results by the three-dimensional ideal MHD simulations are at least qualitatively reproduced These results encourage us to perform this type of a phenomenological simulation to capture a realistic MHD evolution process of the remnant of the binary neutron-star mergers, which cannot be currently studied in the first-principle MHD simulation due to the poor grid resolution resulting from the limitation of the computational resources. II, we suppose to use Cartesian coordinates for the spatial components whenever equations are written
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