We present a numerical study of the hydrodynamics in the final stages of inspiral in a black hole–neutron star binary , when the binary separation becomes comparable to the stellar radius .
We use a Newtonian three–dimensional Smooth Particle Hydrodynamics ( SPH ) code , and model the neutron star with a stiff ( adiabatic index \Gamma = 3 and \Gamma = 2.5 ) polytropic equation of state and the black hole as a Newtonian point mass which accretes matter via an absorbing boundary at the Schwarzschild radius .
Our initial conditions correspond to irrotational binaries in equilibrium ( approximating the neutron star as a compressible tri–axial ellipsoid ) , and we have explored configurations with different values of the initial mass ratio q = M _ { NS } / M _ { BH } , ranging from q = 0.5 to q = 0.2 .
The dynamical evolution is followed using an ideal gas equation of state for approximately 23 ms. We have included gravitational radiation losses in the quadrupole approximation for a point–mass binary .
For the less compressible case ( \Gamma = 3 ) , we find that after an initial episode of intense mass transfer , the neutron star is not completely disrupted and a remnant core remains in orbit about the black hole in a stable binary configuration .
For \Gamma = 2.5 —which is believed to be appropriate for matter at nuclear densities—the tidal disruption process is more complex , with the core of the neutron star surviving the initial mass transfer episode but being totally disrupted during a second periastron passage .
The resulting accretion disc formed around the black hole contains a few tenths of a solar mass .
A nearly baryon–free axis is present in the system throughout the coalescence , and only modest beaming of a relativistic fireball that could give rise to a gamma–ray burst would be sufficient to avoid excessive baryon contamination .
We find that some mass ( on the order of 10 ^ { -2 } M _ { \odot } ) may be dynamically ejected from the system , and could thus contribute substantially to the amount of observed r–process material in the galaxy .
We calculate the gravitational radiation waveforms and luminosity emitted during the coalescence in the quadrupole approximation , and show that they directly reflect the morphology of the coalescence process .
Finally , we present the results of dynamical simulations that have used spherical neutron stars relaxed in isolation as initial conditions , in order to gauge the effect of using non–equilibrium initial conditions on the evolution of the system .