We present numerical results of three-dimensional simulations for the merger of binary neutron stars in full general relativity .
Hybrid equations of state are adopted to mimic realistic nuclear equations of state .
In this approach , we divide the equations of state into two parts as P = P _ { cold } + P _ { th } . P _ { cold } is the cold part for which we assign a fitting formula for realistic equations of state of cold nuclear matter slightly modifying the formula developed by Haensel and Potekhin .
We adopt the SLy and FPS equations of state for which the maximum allowed ADM mass of cold and spherical neutron stars is \approx 2.04 M _ { \odot } and 1.80 M _ { \odot } , respectively . P _ { th } denotes the thermal part which is written as P _ { th } = ( \Gamma _ { th } -1 ) \rho ( \varepsilon - \varepsilon _ { cold } ) , where \rho , \varepsilon , \varepsilon _ { cold } , and \Gamma _ { th } are the baryon rest-mass density , total specific internal energy , specific internal energy of the cold part , and the adiabatic constant , respectively .
Simulations are performed for binary neutron stars of the total ADM mass in the range between 2.4 M _ { \odot } and 2.8 M _ { \odot } with the rest-mass ratio Q _ { M } to be in the range 0.9 \mathrel { \raise 1.29 pt \hbox { $ < $ } \mkern - 14.0 mu \lower 2.58 pt \hbox { $ \sim$ } } Q _ { % M } \leq 1 .
It is found that if the total ADM mass of the system is larger than a threshold M _ { thr } , a black hole is promptly formed in the merger irrespective of the mass ratios .
In the other case , the outcome is a hypermassive neutron star of a large ellipticity , which results from the large adiabatic index of the realistic equations of state adopted .
The value of M _ { thr } depends on the equation of state : M _ { thr } \sim 2.7 M _ { \odot } and \sim 2.5 M _ { \odot } for the SLy and FPS equations of state , respectively .
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 a large amplitude and of frequency between 3 and 4 kHz are emitted .
The estimated emission time scale is \mathrel { \raise 1.29 pt \hbox { $ < $ } \mkern - 14.0 mu \lower 2.58 pt \hbox { $ \sim$ } } 100 ms , after which the hypermassive neutron star collapses to a black hole .
Because of the long emission time , the effective amplitude may be large enough to be detected by advanced laser interferometric gravitational wave detectors if the distance to the source is smaller than \sim 100 Mpc .
Thermal properties of the outcome formed after the merger are also analyzed to approximately estimate the neutrino emission energy .