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

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 aBH/MBH = 0 to aBH/MBH = 0.9 for aligned cases, and we vary the misalignment angle from 0 to 80 • for aBH/MBH = 0.5. We restrict our study to 3:1 mass ratio systems and use a simple Γ-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 • . 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 time scale 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.

Formation of precessing jets by tilted black hole discs in 3D general relativistic MHD simulations

“…In addition to solid-body-like precession, we find that tilted disc-jet systems align with the BH spin axis over longer, accretion timescales (see also Foucart et al 2011). Strong large-scale magnetic fields and the associated powerful jets accelerate this alignment [ Fig.…”

Black-hole–neutron-star mergers at realistic mass ratios: Equation of state and spin orientation effects

“…Within the disk, we still have s $ 4-5. As the disk settles down over $10-20 ms, we would expect the entropy to exhibit a minimum at the peak of the surface density distribution, as was observed in lower mass ratio systems [22,26].…”

“…[26]. Material on an orbit inclined with respect to the equatorial plane of the black hole reaches the region in which stable orbits no longer exist at a larger separation than material in the equatorial plane on a corotating orbit.…”

“…In all cases, the misaligned component of the spin is initially along the line connecting the black-hole and neutron-star centers. We do not expect this choice to affect the qualitative feature of the merger (disruption, disk formation) [26]. It should, however, influence the magnitude of the velocity kick given to the final black hole as a result of gravitational-wave emission [59].…”

“…The only simulations considering mass ratios q ¼ M BH =M NS > 5 have shown that, even for very large neutron stars (R NS $ 14:4 km) and aligned black-hole spins, dimensionless black-hole spins BH ¼ J=M 2 BH * 0:7 are required for tidal disruption to occur [18]. Simulations for more symmetric mass ratios have however already shown that smaller neutron-star radii [22,25] or misaligned black-hole spins [26] are likely to make tidal disruption harder. In this paper, we begin a more quantitative exploration of these effects.…”