Effects of magnetic field topology in black hole-neutron star mergers: Long-term simulations

TL;DR

This study uses long-term simulations to explore how asymmetric magnetic fields in neutron stars affect black hole-neutron star mergers, revealing wind episodes, magnetic field configurations, and turbulence effects in the post-merger evolution.

Contribution

First long-term simulation of black hole-neutron star mergers with asymmetric magnetic fields, analyzing wind launching, magnetic field evolution, and turbulence effects.

Findings

Multiple thermally driven wind episodes observed.

Large-scale poloidal magnetic fields form along funnel walls.

Magnetorotational instability amplifies magnetic fields in the disk.

Abstract

We report long-term simulations of black hole-neutron star binary mergers where the neutron star possesses an asymmetric magnetic field dipole. Focusing on the scenario where the neutron star is tidally disrupted by the black hole, we track the evolution of the binary up to $\approx 100$ms after the merger. We uncover more than one episode of thermally driven winds being launched along a funnel wall in all these cases beginning from $\approx 25$ms after the merger. On the other hand, we are unable to conclude presently whether the amount of ejected mass increases with the \it{degree} of asymmetry. A large-scale magnetic field configuration in the poloidal direction is formed along the funnel wall accompanied by the generation of a large Poynting flux. The magnetic field in the accretion disk around the black hole remnant is amplified by the nonaxisymmetric magnetorotational instability (MRI). The MRI growth is estimated to be in the ideal magnetohydrodynamics (MHD) regime and thus would be free from significant effects induced by potential neutrino radiation. However, the asymmetry in the magnetic field leads to increased turbulence, which causes the vertical magnetic field in the accretion disk to grow largely in a nonlinear manner.