General relativistic simulations of black hole-neutron star mergers: Effects of magnetic fields

TL;DR

This study uses general relativistic magnetohydrodynamic simulations to explore how magnetic fields in neutron stars affect black hole-neutron star mergers, their gravitational wave signals, and potential gamma-ray burst mechanisms.

Contribution

It provides the first detailed GRMHD simulation analysis of magnetic field effects on BH-NS merger dynamics and remnant disk formation.

Findings

Strong magnetic fields significantly increase remnant disk mass.

Magnetic fields may influence gravitational waveforms but are hard to detect.

No conclusive evidence of jet formation or outflows in current simulations.

Abstract

As a neutron star (NS) is tidally disrupted by a black hole (BH) companion at the end of a BH-NS binary inspiral, its magnetic fields will be stretched and amplified. If sufficiently strong, these magnetic fields may impact the gravitational waveforms, merger evolution and mass of the remnant disk. Formation of highly-collimated magnetic field lines in the disk+spinning BH remnant may launch relativistic jets, providing the engine for a short-hard GRB. We analyze this scenario through fully general relativistic, magnetohydrodynamic (GRMHD) BHNS simulations from inspiral through merger and disk formation. Different initial magnetic field configurations and strengths are chosen for the NS interior for both nonspinning and moderately spinning (a/M=0.75) BHs aligned with the orbital angular momentum. Only strong interior (Bmax~10^17 G) initial magnetic fields in the NS significantly influence merger dynamics, enhancing the remnant disk mass by 100% and 40% in the nonspinning and spinning BH cases, respectively. However, detecting the imprint of even a strong magnetic field may be challenging for Advanced LIGO. Though there is no evidence of mass outflows or magnetic field collimation during the preliminary simulations we have performed, higher resolution, coupled with longer disk evolutions and different initial magnetic field configurations, may be required to definitively assess the possibility of BHNS binaries as short-hard GRB progenitors.