Black Hole - Neutron Star Mergers as Central Engines of Gamma-Ray Bursts
Abstract
Hydrodynamic simulations of the merging of stellar mass black hole -- neutron star binaries (BH/NS) are compared with mergers of binary neutron stars (NS/NS). The simulations are Newtonian, but take into account the emission of gravitational waves and their backreaction on the hydrodynamics. The use of a physical nuclear equation of state allowed us to include the effects of neutrino emission by a neutrino trapping scheme. 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 already distroyed during its first approach. A gas mass between about 0.3 and about 0.7 solar masses is left in an accretion torus around the black hole and radiates neutrinos at a luminosity of several 10^{53} erg/s. The emitted neutrinos and antineutrinos annihilate into electron-positron pairs with efficiencies of a few percent and rates of up to about 2*10^{52} erg/s, releasing an estimated energy of up to about 10^{51} erg in a pair-plasma fireball, which could give rise to a gamma-ray burst.Keywords
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