Relativistic Simulations of Black Hole-Neutron Star Mergers: Effects of black-hole spin

Abstract

Black hole-neutron star (BHNS) binary mergers are candidate engines for generating both short-hard gamma-ray bursts (SGRBs) and detectable gravitational waves. Using our most recent conformal thin-sandwich BHNS initial data and our fully general relativistic hydrodynamics code, which is now AMR-capable, we are able to efficiently and accurately simulate these binaries from large separations through inspiral, merger, and ringdown. We evolve the metric using the BSSN formulation with the standard moving puncture gauge conditions and handle the hydrodynamics with a high-resolution shock-capturing scheme. We explore the effects of BH spin (aligned and anti-aligned with the orbital angular momentum) by evolving three sets of initial data with BH:NS mass ratio q=3: the data sets are nearly identical, except the BH spin is varied between a/M = -0.5 (anti-aligned), 0.0, and 0.75. The number of orbits before merger increases with a/M, as expected. We also study the nonspinning BH case in more detail, varying q between 1, 3, and 5. We calculate gravitational waveforms for the cases we simulate and compare them to binary black-hole waveforms. Only a small disk (< 0.01 M_sun) forms for the anti-aligned spin case (a/M = -0.5) and for the most extreme mass ratio case (q=5). By contrast, a massive (M_disk is about 0.2 M_sun), hot disk forms in the rapidly spinning (a/M = 0.75) aligned BH case. Such a disk could drive a SGRB,possibly by, e.g., producing a copious flux of neutrino-antineutino pairs.