Abstract
Long-term ideal and resistive magnetohydrodynamics (MHD) simulations in full general relativity are performed for a massive neutron star formed as a remnant of binary neutron star mergers. Neutrino radiation transport effects are taken into account as in our previous papers. The simulation is performed in axial symmetry and without considering dynamo effects as a first step. In the ideal MHD, the differential rotation of the remnant neutron star amplifies the magnetic-field strength by the winding in the presence of a seed poloidal field until the electromagnetic energy reaches $\sim 10\%$ of the rotational kinetic energy, $E_{\rm kin}$, of the neutron star. The timescale until the maximum electromagnetic energy is reached depends on the initial magnetic-field strength and it is $\sim 1$ s for the case that the initial maximum magnetic-field strength is $\sim 10^{15}$ G. After a significant amplification of the magnetic-field strength by the winding, the magnetic braking enforces the initially differentially rotating state approximately to a rigidly rotating state. In the presence of the resistivity, the amplification is continued only for the resistive timescale, and if the maximum electromagnetic energy reached is smaller than $\sim 3\%$ of $E_{\rm kin}$, the initial differential rotation state is approximately preserved. In the present context, the post-merger mass ejection is induced primarily by the neutrino irradiation/heating and the magnetic winding effect plays only a minor role for the mass ejection.
Highlights
Theoretical exploration for the evolution of the merger remnants of binary neutron stars has become a hot topic in this decade [1], because such systems can shine as an electromagnetic counterpart of gravitational waves emitted in their inspiral stage and the signals bring us a variety of information for the nature of the merger process and neutron stars that cannot be obtained only by the gravitational-wave observation, as the first observation of the binary neutron star merger shows [2,3]
This conclusion is in contrast to previous work [39,40,41], which showed that differentially rotating neutron stars significantly contract, and if they are hypermassive, they collapse to a black hole, after the magnetic winding effect and associated angular momentum transport by the magnetic braking take place
As we reported in our previous papers [11,17,25], the mass ejection proceeds from the remnant of binary neutron star mergers through the neutrino irradiation/heating even in the absence of any other effects like the MHD and viscous effects
Summary
Theoretical exploration for the evolution of the merger remnants of binary neutron stars has become a hot topic in this decade [1], because such systems can shine as an electromagnetic counterpart of gravitational waves emitted in their inspiral stage and the signals bring us a variety of information for the nature of the merger process and neutron stars that cannot be obtained only by the gravitational-wave observation, as the first observation of the binary neutron star merger shows [2,3]. The promising component of the remnant that induces the mass ejection is a disk (or torus) surrounding the central compact object, which is either a massive neutron star or a black hole The reason for this is that the disk is differentially rotating, approximately with the Keplerian rotational profile, and is likely to be magnetized because the disk matter stems from neutron stars. The resulting magnetic pressure could be a substantial fraction of the matter pressure, and the MHD effect could play an important role for the late time evolution of the remnant neutron star. II, we suppose to use Cartesian coordinates for the spatial components whenever equations are written
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