Black hole-neutron star binaries in general relativity: Effects of neutron star spin

We present new sequences of general relativistic, quasiequilibrium black hole-neutron star binaries. We solve for the gravitational field in the conformal thin-sandwich decomposition of Einstein’s field equations, coupled to the equations of relativistic hydrostatic equilibrium for a perfect fluid. We account for the black hole by solving these equations in the background metric of a Schwarzschild black hole whose mass is much larger than that of the neutron star. The background metric is treated in Kerr-Schild as well as isotropic coordinates. For the neutron star, we assume a polytropic equation of state with adiabatic index Γ=2, and solve for both irrotational and corotational configurations. By comparing the results of irrotational and synchronized configurations with the same background metric, we conclude that the effect of the rotation on the location of tidal breakup is only on the order of a few percent. The different choices in the background also lead to differences on the order of a few percent, which may be an indication of the level to which these configurations approximate quasiequilibrium.