General relativistic simulations for the merger of binary neutron stars are performed as an extension of a previous work ( ( 1 ) ) .
We prepare binary neutron stars with a large initial orbital separation and employ the moving-puncture formulation , which enables to follow merger and ringdown phases for a long time , even after black hole formation .
For modeling inspiraling neutron stars , which should be composed of cold neutron stars , the Akmal-Pandharipande-Ravenhall ( APR ) equation of state ( EOS ) is adopted .
After the onset of merger , the hybrid-type EOS is used ; i.e. , the cold and thermal parts are given by the APR and \Gamma -law EOSs , respectively .
Three equal-mass binaries , each with mass 1.4 M _ { \odot } , 1.45 M _ { \odot } , and 1.5 M _ { \odot } , and two unequal-mass binaries with mass , 1.3 vs 1.6 M _ { \odot } and 1.35 vs 1.65 M _ { \odot } , are prepared .
We focus primarily on the black hole formation case , and explore mass and spin of the black hole , mass of disks which surround the black hole , and gravitational waves emitted during the black hole formation .
We find that ( i ) the black hole is promptly formed if total mass of the system initially satisfies m _ { 0 } \gtrsim 2.9 M _ { \odot } ; ( ii ) for the systems of m _ { 0 } = 2.9 – 3.0 M _ { \odot } and of mass ratio \approx 0.8 , the mass of disks which surround the formed black hole is 0.006– 0.02 M _ { \odot } ; ( iii ) the spin of the formed black hole is 0.78 \pm 0.02 when a black hole is formed after the merger in the dynamical time scale .
This value depends weakly on the total mass and mass ratio , and is about 0.1 larger than that of a black hole formed from nonspinning binary black holes ; ( iv ) for the black-hole formation case , Fourier spectrum shape of gravitational waves emitted in the merger and ringdown phases has a universal qualitative feature irrespective of the total mass and mass ratio , but quantitatively , the spectrum reflects the parameters of the binary neutron stars .