Using our new numerical-relativity code SACRA , long-term simulations for inspiral and merger of black hole ( BH ) -neutron star ( NS ) binaries are performed , focusing particularly on gravitational waveforms .
As the initial conditions , BH-NS binaries in a quasiequilibrium state are prepared in a modified version of the moving-puncture approach .
The BH is modeled by a nonspinning moving puncture and for the NS , a polytropic equation of state with \Gamma = 2 and the irrotational velocity field are employed .
The mass ratio of the BH to the NS , Q = M _ { BH } / M _ { NS } , is chosen in the range between 1.5 and 5 .
The compactness of the NS , defined by { \cal C } = GM _ { NS } / c ^ { 2 } R _ { NS } , is chosen to be between 0.145 and 0.178 .
For a large value of Q for which the NS is not tidally disrupted and is simply swallowed by the BH , gravitational waves are characterized by inspiral , merger , and ringdown waveforms .
In this case , the waveforms are qualitatively the same as that from BH-BH binaries .
For a sufficiently small value of Q \lesssim 2 , the NS may be tidally disrupted before it is swallowed by the BH .
In this case , the amplitude of the merger and ringdown waveforms is very low , and thus , gravitational waves are characterized by the inspiral waveform and subsequent quick damping .
The difference in the merger and ringdown waveforms is clearly reflected in the spectrum shape and in the “ cut-off ” frequency above which the spectrum amplitude steeply decreases .
When an NS is not tidally disrupted ( e.g. , for Q = 5 ) , kick velocity , induced by asymmetric gravitational wave emission , agrees approximately with that derived for the merger of BH-BH binaries , whereas for the case that the tidal disruption occurs , the kick velocity is significantly suppressed .