Merger of binary neutron stars to a black hole: Disk mass, short gamma-ray bursts, and quasinormal mode ringing

Abstract

Three-dimensional simulations for the merger of binary neutron stars (BNSs) are performed in the framework of full general relativity. We pay particular attention to the black hole (BH) formation case and to the resulting mass of the surrounding disk for exploring possibility for formation of the central engine of short-duration gamma-ray bursts. Hybrid equations of state (EOSs) are adopted mimicking realistic, stiff nuclear EOSs, for which the maximum allowed gravitational mass of cold and spherical neutron stars (NSs), M_sph, is larger than 2M_sun. For the simulations, we focus on BNSs of the ADM mass M>2.6M_sun. For M>M_thr, the merger results in prompt formation of a BH irrespective of the mass ratio Q_M with 0.65 0.01M_sun. Gravitational waves (GWs) are computed in terms of a gauge-invariant wave extraction technique. In the formation of the HMNS, quasiperiodic GWs of frequency (3-3.5kHz) are emitted. The effective amplitude of GWs can be >5x10^{-21} at a distance of 50 Mpc. For the BH formation case, the BH excision technique enables a longterm computation and extraction of ring-down GWs associated with a BH quasinormal mode. It is found that the frequency and amplitude are 6.5-7kHz and 10^{-22} at a distance of 50Mpc for M=2.7-2.9M_sun.