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

TL;DR

This paper presents full general relativistic simulations of black hole-neutron star mergers, analyzing tidal disruption, torus formation, and gravitational wave emission for various initial conditions.

Contribution

It provides the first systematic simulation results of BH-NS mergers focusing on tidal disruption, torus mass, and gravitational waveforms in full general relativity.

Findings

Tidal disruption occurs near the innermost stable circular orbit for all cases.

Torus mass varies significantly with neutron star radius, from less than 0.04 to about 0.16 solar masses.

Gravitational waveforms capture the inspiral, disruption, and post-merger evolution.

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 ..)