Long-term general relativistic simulation of binary neutron stars collapsing to a black hole

Abstract

General relativistic simulations for the merger of binary neutron stars are performed as an extension of a previous work [M. Shibata and K. Taniguchi, Phys. Rev. D 73, 064027 (2006).]. We prepare binary neutron stars with a large initial orbital separation and employ the moving-puncture formulation, which enables one 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 $\ensuremath{\Gamma}$-law EOSs, respectively. Three equal-mass binaries, each with mass $1.4{M}_{\ensuremath{\bigodot}}$, $1.45{M}_{\ensuremath{\bigodot}}$, and $1.5{M}_{\ensuremath{\bigodot}}$, and two unequal-mass binaries with mass, 1.3 vs $1.6{M}_{\ensuremath{\bigodot}}$ and 1.35 vs $1.65{M}_{\ensuremath{\bigodot}}$, 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 ${m}_{0}\ensuremath{\gtrsim}2.9{M}_{\ensuremath{\bigodot}}$; (ii) for the systems of ${m}_{0}=2.9--3.0{M}_{\ensuremath{\bigodot}}$ and of mass ratio $\ensuremath{\approx}0.8$, the mass of disks which surround the formed black hole is $0.006--0.02{M}_{\ensuremath{\bigodot}}$; (iii) the spin of the formed black hole is $0.78\ifmmode\pm\else\textpm\fi{}0.02$ 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.