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 \leq 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 } \approx 0.2 M _ { \odot } ) , with baryon–free regions that could possibly give rise to gamma ray bursts .