Three-dimensional simulations for the merger of binary neutron stars are performed in the framework of full general relativity .
We pay particular attention to the black hole 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 ( SGRBs ) .
Hybrid equations of state are adopted mimicking realistic , stiff nuclear equations of state ( EOSs ) , for which the maximum allowed gravitational mass of cold and spherical neutron stars , M _ { sph } , is larger than 2 M _ { \odot } .
Such stiff EOSs are adopted motivated by the recent possible discovery of a heavy neutron star of mass \sim 2.1 \pm 0.2 M _ { \odot } .
For the simulations , we focus on binary neutron stars of the ADM mass M \mathrel { \raise 1.29 pt \hbox { $ > $ } \mkern - 14.0 mu \lower 2.58 pt \hbox { $ \sim$ } } 2.6 M _ % { \odot } .
For an ADM mass larger than the threshold mass M _ { thr } , the merger results in prompt formation of a black hole irrespective of the mass ratio Q _ { M } with 0.65 \mathrel { \raise 1.29 pt \hbox { $ < $ } \mkern - 14.0 mu \lower 2.58 pt \hbox { $ \sim$ } } Q _ % { M } \leq 1 .
The value of M _ { thr } depends on the EOSs and is approximately written as 1.3 – 1.35 M _ { sph } for the chosen EOSs .
For the black hole formation case , we evolve the spacetime using a black hole excision technique and determine the mass of a quasistationary disk surrounding the black hole .
The disk mass steeply increases with decreasing the value of Q _ { M } for given ADM mass and EOS .
This suggests that a merger with small value of Q _ { M } is a candidate for producing central engine of SGRBs .
For M < M _ { thr } , the outcome is a hypermassive neutron star of a large ellipticity .
Because of the nonaxisymmetry , angular momentum is transported outward .
If the hypermassive neutron star collapses to a black hole after the longterm angular momentum transport , the disk mass may be \mathrel { \raise 1.29 pt \hbox { $ > $ } \mkern - 14.0 mu \lower 2.58 pt \hbox { $ \sim$ } } 0.01 M _ % { \odot } irrespective of Q _ { M } .
Gravitational waves are computed in terms of a gauge-invariant wave extraction technique .
In the formation of the hypermassive neutron star , quasiperiodic gravitational waves of frequency between 3 and 3.5 kHz are emitted irrespective of EOSs .
The effective amplitude of gravitational waves can be \mathrel { \raise 1.29 pt \hbox { $ > $ } \mkern - 14.0 mu \lower 2.58 pt \hbox { $ \sim$ } } 5 % \times 10 ^ { -21 } at a distance of 50 Mpc , and hence , it may be detected by advanced laser-interferometers .
For the black hole formation case , the black hole excision technique enables a longterm computation and extraction of ring-down gravitational waves associated with a black hole quasinormal mode .
It is found that the frequency and amplitude are \approx 6.5 –7 kHz and \sim 10 ^ { -22 } at a distance of 50 Mpc for the binary of mass M \approx 2.7 – 2.9 M _ { \odot } .