General-Relativistic Simulations of Black Hole-Neutron Star Mergers: Effects of Tilted Magnetic Fields

University of Illinois at Urbana-Champaign


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 ~ 108 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., 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 sufficent to power an SGRB.


[PRD 86, 084026, (2012)]


For this simulation we use both 2D and 3D rendering to illustrate our data. When comparing the two, there are two important things to consider: spacial mappings and color mappings. In the 2D view, each point on the screen maps directly to a point in the plane. The value of the mass per volume at that point corresponds to a specific color of our choice. In volumetric 3D rendering, we integrate rays from the observer through a viewing plane. Each point on that viewing plane corresponds to a point on the screen, but this time we consider all points along the ray. Each ray gives a mass per area which corresponds to a specific color of our choice. The result reveals the structure of a density distribution in 3D. Note the difference between this and the way we perceive clouds is in the way light scatters in each volume element, giving bright and shadowy areas in real life.

These visualizations were created using the ZIB Amira software package [D. Stalling, M. Westerhoff, and H.-C. Hege, in The Visualization Handbook; Amira: A Highly Interactive System for Visual Data Analysis, edited by C. D. Hansen and C. R. Johnson (Elsevier, 2005) Chap. 38, pp. 749-767, ISBN 978-0-12-387582-2]. We gratefully acknowledge ZIB for providing us a license.

University of Illinois at Urbana-Champaign

Part I - Evolution with Tilted Magnetic Fields

Initial Configuration
Density Profile
Entropy Profile
Gravitational Waveforms

Part II - Seeding the Remnant Disk with a Poloidal Magnetic Field

Initial Configuration
3D Density Profile
2D Density Profile
Magnetic to Rest-Mass Density Ratio