Merger of black hole and neutron star in general relativity: Tidal disruption, torus mass, and gravitational waves

Abstract

We systematically perform the merger simulation of black hole-neutron star (BH-NS) binaries in full general relativity, focusing on the case that the NS is tidally disrupted. We prepare BH-NS binaries in a quasicircular orbit as the initial condition in which the BH is modeled by a nonspinning moving puncture. For modeling the NS, we adopt the $\Gamma$-law equation of state with $\Gamma=2$ and the irrotational velocity field. We change the BH mass in the range $M_{\rm BH} \approx 3.3$--$4.6M_{\odot}$, while the rest mass of the NS is fixed to be $M_{*}=1.4 M_{\odot}$ (i.e., the NS mass $M_{\rm NS} \approx 1.3M_{\odot}$). The radius of the corresponding spherical NS is set in the range $R_{\rm NS} \approx 12$--15 km (i.e., the compactness $GM_{\rm NS}/R_{\rm NS}c^2 \approx 0.13$--0.16). We find for all the chosen initial conditions that the NS is tidally disrupted near the innermost stable circular orbit. For the model of $R_{\rm NS}=12$ km, more than 97 % of the rest mass is quickly swallowed into the BH and the resultant torus mass surrounding the BH is less than $0.04M_{\odot}$. For the model of $R_{\rm NS} \approx 14.7$ km, by contrast, the torus mass is about $0.16M_{\odot}$ for the BH mass $\approx 4M_{\odot}$. The thermal energy of the material in the torus increases by the shock heating occurred in the collision between the spiral arms, resulting in the temperature $10^{10}$--$10^{11}$ K. (.. omission ..) We also present gravitational waveforms during the inspiral, tidal disruption of the NS, and subsequent evolution of the disrupted material. (.. omission ..)