# Gravitational waves from black hole-neutron star binaries I: Classification of waveforms - General Relativity and Quantum Cosmology

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Abstract: Using our new numerical-relativity code SACRA, long-term simulations forinspiral and merger of black hole BH-neutron star NS binaries areperformed, focusing particularly on gravitational waveforms. As the initialconditions, BH-NS binaries in a quasiequilibrium state are prepared in amodified version of the moving-puncture approach. The BH is modeled by anonspinning moving puncture and for the NS, a polytropic equation of state with$\Gamma=2$ and the irrotational velocity field are employed. The mass ratio ofthe BH to the NS, $Q=M { m BH}-M { m NS}$, is chosen in the range between 1.5and 5. The compactness of the NS, defined by ${\cal C}=GM { m NS}-c^2R { mNS}$, is chosen to be between 0.145 and 0.178. For a large value of $Q$ forwhich the NS is not tidally disrupted and is simply swallowed by the BH,gravitational waves are characterized by inspiral, merger, and ringdownwaveforms. In this case, the waveforms are qualitatively the same as that fromBH-BH binaries. For a sufficiently small value of $Q \alt 2$, the NS may betidally disrupted before it is swallowed by the BH. In this case, the amplitudeof the merger and ringdown waveforms is very low, and thus, gravitational wavesare characterized by the inspiral waveform and subsequent quick damping. Thedifference in the merger and ringdown waveforms is clearly reflected in thespectrum shape and in the -cut-off- frequency above which the spectrumamplitude steeply decreases. When an NS is not tidally disrupted e.g., forQ=5, kick velocity, induced by asymmetric gravitational wave emission, agreesapproximately with that derived for the merger of BH-BH binaries, whereas forthe case that the tidal disruption occurs, the kick velocity is significantlysuppressed.

Author: Masaru Shibata, Koutarou Kyutoku, Tetsuro Yamamoto, Keisuke Taniguchi

Source: https://arxiv.org/