Black hole-neutron star mergers: effects of the orientation of the black hole spin

TL;DR

This study uses full general relativity simulations to explore how the orientation of black hole spin affects the dynamics and postmerger disk properties in black hole-neutron star mergers, revealing significant effects at high misalignment angles.

Contribution

First full GR simulations of BHNS mergers with misaligned black hole spins, analyzing effects on disk mass, tilt, and potential for gamma-ray burst production.

Findings

Misalignment angle > 40 degrees significantly reduces disk mass.

Disk lifetime remains around 100ms regardless of spin and tilt.

Disks are hot, thick, and suitable for gamma-ray bursts.

Abstract

The spin of black holes in black hole-neutron star (BHNS) binaries can have a strong influence on the merger dynamics and the postmerger state; a wide variety of spin magnitudes and orientations are expected to occur in nature. In this paper, we report the first simulations in full general relativity of BHNS mergers with misaligned black hole spin. We vary the spin magnitude from a/m=0 to a/m=0.9 for aligned cases, and we vary the misalignment angle from 0 to 80 degrees for a/m=0.5. We restrict our study to 3:1 mass ratio systems and use a simple Gamma-law equation of state. We find that the misalignment angle has a strong effect on the mass of the postmerger accretion disk, but only for angles greater than ~ 40 degrees. Although the disk mass varies significantly with spin magnitude and misalignment angle, we find that all disks have very similar lifetimes ~ 100ms. Their thermal and rotational profiles are also very similar. For a misaligned merger, the disk is tilted with respect to the final black hole's spin axis. This will cause the disk to precess, but on a timescale longer than the accretion time. In all cases, we find promising setups for gamma-ray burst production: the disks are hot, thick, and hyperaccreting, and a baryon-clear region exists above the black hole.