Gravitational waves from black hole-neutron star binaries: Classification of waveforms

Abstract

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 Γ = 2 and the irrotational velocity field are employed. The mass ratio of the BH to the NS, Q = MBH/MNS, is chosen in the range between 1.5 and 5. The compactness of the NS, defined by C = GMNS/c 2 RNS, 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 < ∼ 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.

Polytropic neutron star – black hole merger simulations with a Paczyński-Wiita potential

“…However, we cannot identify a clear step change. This lack of clear demarcation between ISCO and MS is also the case for the models including general relativity (Shibata et al 2009, Fig. 3).…”

“…Thus the questions of how much matter is accreted, how much is ejected, and how much forms a torus, need to be investigated carefully. In this point, the numerical general relativistic simulations (Rantsiou et al 2008;Etienne et al 2009;Shibata et al 2009, their Fig. 7) still differ markedly among themselves, even for models with similar parameters.…”

“…They are also shown in Fig. 2 and were chosen to match closely the models presented in Shibata et al (2009) (see Fig. 3).…”

“…We included detailed microphysics (equation of state, neutrino source terms) but accounted for general relativistic effects either only partially (gravitational wave emission and backreaction) or phenomenologically (Paczyński-Wiita potential for the BH). Other groups interested in gravitational wave aspects have published results of NS-BH mergers with general relativistic (GR) physics included to different levels of approximation (Etienne et al 2009;Shibata et al 2009;Rantsiou et al 2008;Taniguchi et al 2008;Duez et al 2008), but with some variant of the polytropic equation-of-state (EoS) for the neutron star matter. Rantsiou et al (2008) did not include dynamic GR in that they used a fixed background.…”

Bowen-York-type initial data for binaries with neutron stars

“…The coordinate separation between the BH and the NS is 117 km. With these parameters, the BH-NS binary is similar to the M50.145b system in Shibata et al [48]. As with the DNS system, we cover the star with a single mesh whose side length is the diameter of the star.…”