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

TL;DR

This study shows that tilted magnetic fields in black hole-neutron star mergers enhance poloidal magnetic field generation, enabling MRI-driven turbulence and jet formation, which are crucial for short gamma-ray burst engines.

Contribution

It demonstrates that asymmetric initial magnetic field configurations can produce stronger poloidal fields, facilitating MRI and jet formation in fully relativistic MHD simulations of BHNS mergers.

Findings

Tilted magnetic fields generate stronger poloidal fields in the disk.

MRI-driven turbulence supports accretion and jet outflows.

Asymmetric initial conditions may be essential for SGRB progenitors.

Abstract

Black hole--neutron star (BHNS) binary mergers can form disks in which magnetorotational instability (MRI)-induced turbulence may drive accretion onto the remnant BH, supporting relativistic jets and providing the engine for a short-hard gamma-ray burst (SGRB). Our earlier study of magnetized BHNSs showed that NS tidal disruption winds the magnetic field into a toroidal configuration, with poloidal fields so weak that capturing MRI with full-disk simulations would require $\sim 10^8$ CPU-hours. In that study we imposed equatorial symmetry, suppressing poloidal magnetic fields that might be generated from plasma crossing the orbital plane. Here we show that initial conditions that break this symmetry (i.e., {\it tilted} poloidal magnetic fields in the NS) generate much stronger poloidal fields in the disk, indicating that asymmetric initial conditions may be necessary for establishing BHNS mergers as SGRB progenitors via fully general relativistic MHD simulations. We demonstrate that BHNS mergers may form an SGRB engine under the right conditions by seeding the remnant disk from an unmagnetized BHNS simulation with purely poloidal fields dynamically unimportant initially, but strong enough to resolve MRI. Magnetic turbulence occurs in the disk, driving accretion and supporting Poynting-dominated jet outflows sufficient to power an SGRB.