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

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.

💡 Research Summary

This paper investigates whether black‑hole–neutron‑star (BH‑NS) binary mergers can serve as the central engine of short‑hard gamma‑ray bursts (SGRBs) by performing fully general‑relativistic magnetohydrodynamic (GRMHD) simulations that focus on the role of magnetic‑field geometry. In earlier work the authors imposed equatorial symmetry on the initial neutron‑star (NS) magnetic field, which forced the post‑merger accretion disk to develop a predominantly toroidal field while the poloidal component became so weak that resolving the magnetorotational instability (MRI) would have required on the order of 10⁸ CPU‑hours—far beyond practical limits.

The present study relaxes that symmetry by initializing the NS with a tilted poloidal magnetic field (tilt angles of 45° and 90° were examined). This asymmetry allows plasma that crosses the orbital plane during tidal disruption to generate additional poloidal flux through induction. The simulations, carried out with a 3‑D GRMHD code for a range of mass ratios, black‑hole spins, and magnetic‑field strengths, reveal several key outcomes. First, the tilted field dramatically amplifies the poloidal component in the remnant disk, increasing it by roughly one to two orders of magnitude relative to the symmetric case. The resulting plasma β (gas‑pressure‑to‑magnetic‑pressure ratio) falls into the range 10–100, a regime in which the fastest‑growing MRI wavelength is well resolved on the computational grid.

Second, the MRI quickly becomes active throughout the disk, driving vigorous magnetohydrodynamic turbulence. This turbulence transports angular momentum outward efficiently, sustaining a mass‑accretion rate onto the black hole of ∼0.01–0.03 M⊙ s⁻¹. The turbulent stresses also maintain a quasi‑steady magnetic pressure that prevents the disk from collapsing into a purely toroidal configuration.

Third, the amplified poloidal field threads the spinning black hole and launches a Poynting‑dominated jet along the rotation axis. The jet power reaches ≈10⁵¹ erg s⁻¹, comparable to the isotropic‑equivalent luminosities inferred for observed SGRBs. The jet initially has a narrow opening angle (~10°) but widens as it interacts with the surrounding outflow, producing a collimated, relativistic outflow capable of powering a short‑duration gamma‑ray burst.

To test the robustness of this mechanism, the authors also performed a “seeding” experiment: they took an otherwise unmagnetized BH‑NS merger simulation and inserted a weak, purely poloidal field into the post‑merger disk. Even when the seed field was dynamically negligible at insertion, it grew under MRI to the same strength as in the tilted‑field runs, leading to comparable turbulence and jet formation. This demonstrates that a modest, non‑axisymmetric magnetic component can be sufficient to trigger the full SGRB engine, provided the disk geometry permits MRI growth.

The paper concludes that breaking equatorial symmetry in the initial magnetic configuration is likely a necessary condition for BH‑NS mergers to become viable SGRB progenitors in fully GRMHD simulations. Real neutron stars are expected to possess complex, possibly misaligned magnetic fields, so the tilted‑field scenario may be astrophysically realistic. Future work should explore a broader parameter space (different tilt angles, black‑hole spins, NS magnetic topologies) and incorporate radiative transfer and neutrino physics to connect the simulated jet properties directly with observable SGRB signatures. Such comprehensive studies will be essential to confirm whether BH‑NS mergers can indeed account for a significant fraction of the short‑hard gamma‑ray burst population.