Black hole-neutron star mergers as central engines of gamma-ray bursts

Hydrodynamic simulations of the merger of stellar mass black hole-neutron s tar binaries are compared with mergers of binary neutron stars. The simulat ions 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-bl ack hale mass ratios, the neutron star transfers mass to the black hole dur ing a few cycles of orbital decay and subsequent widening before finally be ing disrupted, whereas for ratios near unity the neutron star is destroyed during its first approach. A gas mass between similar to 0.3 and similar to 0.7 M. is left in an accretion torus around the black hole and radiates ne utrinos at a luminosity of several times 10(53) ergs s(-1) during an estima ted accretion timescale of about 0.1 a. The emitted neutrinos and antineutr inos annihilate into e(+/-) pairs with efficiencies of 1%-3% and rates of u p to similar to 2 x 10(52) ergs s(-1), thus depositing an energy E-nu<(nu)o ver bar> less than or similar to 10(51) ergs above the poles of the black h ole 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 L-gamma similar to E-n u<(nu)over bar>(f(Omega)t(gamma)) up to several times 10(53) ergs s(-1) for burst durations t(gamma) approximate to 0.1-1 s, if the gamma emission is collimated in two moderately focused jets in a fraction f(Omega) = 2 delta Omega/(4 pi) approximate to (1/100)-(1/10) of the sky.