Simulations of black hole-neutron star binary coalescence

Abstract

We show the results of dynamical simulations of the coalescence of black hole-neutron star binaries. We use a Newtonian Smooth Particle Hydrodynamics code, and include the effects of gravitational radiation back reaction with the quadrupole approximation for point masses, and compute the gravitational radiation waveforms. We assume a polytropic equation of state determines the structure of the neutron star in equilibrium, and use an ideal gas law to follow the dynamical evolution. Three main parameters are explored: (i) The distribution of angular momentum in the system in the initial configuration, namely tidally locked systems vs. irrotational binaries; (ii) The stiffness of the equation of state through the value of the adiabatic index Gamma (ranging from Gamma=5/3 to Gamma=3); (iii) The initial mass ratio q=M(NS)/M(BH). We find that it is the value of Gamma that determines how the coalescence takes place, with immediate and complete tidal disruption for Gamma less than 2, while the core of the neutron star survives and stays in orbit around the black hole for Gamma=3. This result is largely independent of the initial mass ratio and spin configuration, and is reflected directly in the gravitational radiation signal. For a wide range of mass ratios, massive accretion disks are formed (M(disk)~0.2 solar masses), with baryon-free regions that could possibly give rise to gamma ray bursts.