Mergers of Black Hole -- Neutron Star binaries. I. Methods and First Results

Abstract

We use a 3-D relativistic SPH (Smoothed Particle Hydrodynamics) code to study mergers of black hole -- neutron star (BH--NS) binary systems with low mass ratios, adopting $M_{NS}/M_{BH}\simeq 0.1$ as a representative case. The outcome of such mergers depends sensitively on both the magnitude of the BH spin and its obliquity (i.e., the inclination of the binary orbit with respect to the equatorial plane of the BH). In particular, only systems with sufficiently high BH spin parameter $a$ and sufficiently low orbital inclinations allow any NS matter to escape or to form a long-lived disk outside the BH horizon after disruption. Mergers of binaries with orbital inclinations above $\sim 60^o$ lead to complete prompt accretion of the entire NS by the BH, even for the case of an extreme Kerr BH. We find that the formation of a significant disk or torus of NS material around the BH always requires a near-maximal BH spin and a low initial inclination of the NS orbit just prior to merger.

to appear in ApJ, 54 pages, 19 figures

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• Populating the Galaxy with pulsars -- II. Galactic dynamics
• Evolution of near-extremal-spin black holes using the moving puncture technique
• Multimessenger Binary Mergers Containing Neutron Stars: Gravitational Waves, Jets, and $\mathbf{\gamma }$-Ray Bursts
• First eccentric inspiral-merger-ringdown analysis of neutron star-black hole mergers
• Comparison of the Origin of Short Gamma-Ray Bursts with or without Extended Emission