General-relativistic resistive-magnetohydrodynamic simulations of binary neutron stars

TL;DR

This study compares resistive and ideal magnetohydrodynamic simulations of binary neutron star mergers, revealing resistive effects prolong the hypermassive neutron star phase due to less efficient magnetic braking.

Contribution

It provides the first detailed resistive MHD simulation of binary neutron star mergers, highlighting differences from ideal MHD in the post-merger evolution.

Findings

Resistive MHD extends the survival time of hypermassive neutron stars.

Magnetic braking is less efficient in resistive MHD, affecting angular momentum transport.

Both regimes produce a low-density funnel with ordered magnetic fields.

Abstract

We have studied the dynamics of an equal-mass magnetized neutron-star binary within a resistive magnetohydrodynamic (RMHD) approach in which the highly conducting stellar interior is matched to an electrovacuum exterior. Because our analysis is aimed at assessing the modifications introduced by resistive effects on the dynamics of the binary after the merger and through to collapse, we have carried out a close comparison with an equivalent simulation performed within the traditional ideal magnetohydrodynamic approximation. We have found that there are many similarities between the two evolutions but also one important difference: the survival time of the hyper massive neutron star increases in a RMHD simulation. This difference is due to a less efficient magnetic-braking mechanism in the resistive regime, in which matter can move across magnetic-field lines, thus reducing the outward transport of angular momentum. Both the RMHD and the ideal magnetohydrodynamic simulations carried here have been performed at higher resolutions and with a different grid structure than those in previous work of ours [L. Rezzolla, B. Giacomazzo, L. Baiotti, J. Granot, C. Kouveliotou, and M. A. Aloy, Astrophys. J. Letters 732, L6 (2011)], but confirm the formation of a low-density funnel with an ordered magnetic field produced by the black hole--torus system. In both regimes the magnetic field is predominantly toroidal in the highly conducting torus and predominantly poloidal in the nearly evacuated funnel. Reconnection processes or neutrino annihilation occurring in the funnel, none of which we model, could potentially increase the internal energy in the funnel and launch a relativistic outflow, which, however, is not produced in these simulations.