General relativistic simulations of black-hole–neutron-star mergers: Effects of black-hole spin

Black-hole--neutron-star (BHNS) binary mergers are candidate engines for generating both short-hard gamma-ray bursts and detectable gravitational waves. Using our most recent conformal thin-sandwich BHNS initial data and our fully general relativistic hydrodynamics code, which is now adaptive mesh refinement 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 Baumgarte-Shapiro-Shibata-Nakamura 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 antialigned 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 $a/{M}_{\mathrm{BH}}=\ensuremath{-}0.5$ (antialigned), 0.0, and 0.75. The number of orbits before merger increases with $a/{M}_{\mathrm{BH}}$, 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 ($<0.01{M}_{\ensuremath{\bigodot}}$) forms for the antialigned spin case ($a/{M}_{\mathrm{BH}}=\ensuremath{-}0.5$) and for the most extreme-mass-ratio case ($q=5$). By contrast, a massive (${M}_{\mathrm{disk}}\ensuremath{\approx}0.2{M}_{\ensuremath{\bigodot}}$) hot disk forms in the rapidly spinning ($a/{M}_{\mathrm{BH}}=0.75$) aligned BH case. Such a disk could drive a short-hard gamma-ray burst, possibly by, e.g., producing a copious flux of neutrino-antineutrino pairs.