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 binaries can have a strong influence on the merger dynamics and the post-merger 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 black hole-neutron star mergers with misaligned black hole spin. We vary the spin magnitude from ${a}_{\mathrm{BH}}/{M}_{\mathrm{BH}}=0$ to ${a}_{\mathrm{BH}}/{M}_{\mathrm{BH}}=0.9$ for aligned cases, and we vary the misalignment angle from 0 to 80\ifmmode^\circ\else\textdegree\fi{} for ${a}_{\mathrm{BH}}/{M}_{\mathrm{BH}}=0.5$. We restrict our study to $3\ensuremath{\mathbin:}1$ mass-ratio systems and use a simple $\ensuremath{\Gamma}$-law equation of state. We find that the misalignment angle has a strong effect on the mass of the post-merger accretion disk, but only for angles greater than $\ensuremath{\approx}40\ifmmode^\circ\else\textdegree\fi{}$. Although the disk mass varies significantly with spin magnitude and misalignment angle, we find that all disks have very similar lifetimes $\ensuremath{\approx}100\text{ }\text{ }\mathrm{ms}$. 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.