Binary neutron star mergers: Dependence on the nuclear equation of state

Abstract

We perform a numerical-relativity simulation for the merger of binary neutron stars with 6 nucleartheory-based equations of state (EOSs) described by piecewise polytropes. Our purpose is to explore the dependence of the dynamical behavior of the binary neutron star merger and resulting gravitational waveforms on the EOS of the supernuclear-density matter. The numerical results show that the merger process and the first outcome are classified into three types; (i) a black hole is promptly formed, (ii) a short-lived hypermassive neutron star (HMNS) is formed, (iii) a long-lived HMNS is formed. The type of the merger depends strongly on the EOS and on the total mass of the binaries. For the EOS with which the maximum mass is larger than 2M⊙, the lifetime of the HMNS is longer than 10 ms for a total mass m0 = 2.7M⊙. A recent radio observation suggests that the maximum mass of spherical neutron stars is Mmax ≥ 1.97 ± 0.04M⊙ in one σ level. This fact and our results support the possible existence of a HMNS soon after the onset of the merger for a typical binary neutron star with m0 = 2.7M⊙. We also show that the torus mass surrounding the remnant black hole is correlated with the type of the merger process; the torus mass could be large, ≥ 0.1M⊙, in the case that a long-lived HMNS is formed. We also show that gravitational waves carry information of the merger process, the remnant, and the torus mass surrounding a black hole.

Black-hole–neutron-star mergers at realistic mass ratios: Equation of state and spin orientation effects

“…General relativistic simulations of BNS systems have found that only a small amount of mass is unbound by the merger (M ej ¼ 10 À4 M -10 À2 M ) [19], while Newtonian smoothed particle hydrodynamics simulations have more optimistic predictions (M ej * 10 À2 M ) [20]. BHNS systems have more asymmetric mass ratios, and are thus generally more favorable for the ejection of neutronrich material during the disruption of the neutron star.…”

Exploring Physics of Neutron Star Matter by Gravitational Waves

“…Although the total mass of the system exceeds the maximum mass of the static spherically symmetric NS, the merger remnant does not necessarily collapse into a black hole immediately. This is because the maximum mass is increased by ∆M ∼ several 10 % by the thermal and rotational effects [30]. If the total mass of NS-NS is larger than this effective critical mass, a black hole is immediately formed and the amplitude of GW decays rapidly.…”

Prospects of Constraining the Nuclear Equation of State with Gravitational-Wave Signals in the Advanced Detector Era and Beyond

“…Hotokezaka et al [17] shows that there are three types of post-merger behaviour of binary neutron stars, namely prompt formation of black hole, formation of short-lived hyper-massive neutron star and formation of long-lived hyper-massive neutron star. There is a distinct amplitude peak at the spectrum of the gravitational-wave waveform of the post-merger stage.…”