Black Hole–Neutron Star Mergers as Central Engines of Gamma-Ray Bursts

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Abstract
Hydrodynamic simulations of the merger of stellar mass black hole-neutron star binaries are compared with mergers of binary neutron stars. The simulations are Newtonian but take into account the emission and back-reaction of gravitational waves. The use of a physical nuclear equation of state allows us to include the effects of neutrino emission. For low neutron star-to-black hole mass ratios, the neutron star transfers mass to the black hole during a few cycles of orbital decay and subsequent widening before finally being disrupted, whereas for ratios near unity the neutron star is destroyed during its first approach. A gas mass between ~0.3 and ~0.7 M☉ is left in an accretion torus around the black hole and radiates neutrinos at a luminosity of several times 1053 ergs s-1 during an estimated accretion timescale of about 0.1 s. The emitted neutrinos and antineutrinos annihilate into e± pairs with efficiencies of 1%-3% and rates of up to ~2 × 1052 ergs s-1, thus depositing an energy E 1051 ergs above the poles of the black hole in a region that contains less than 10-5 M☉ of baryonic matter. This could allow for relativistic expansion with Lorentz factors around 100 and is sufficient to explain apparent burst luminosities Lγ ~ E/(fΩtγ) up to several times 1053 ergs s-1 for burst durations tγ ≈ 0.1-1 s, if the γ emission is collimated in two moderately focused jets in a fraction fΩ = 2δΩ/(4π) ≈ -(1/10) of the sky.