Merger of binary neutron stars to a black hole: Disk mass, short gamma-ray bursts, and quasinormal mode ringing

Abstract

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 2M⊙. Such stiff EOSs are adopted motivated by the recent possible discovery of a heavy neutron star of mass ∼ 2.1 ± 0.2M⊙. For the simulations, we focus on binary neutron stars of the ADM mass M > ∼ 2.6M⊙. 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 QM with 0.65 < ∼ QM ≤ 1. The value of M thr depends on the EOSs and is approximately written as 1.3-1.35M 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 QM for given ADM mass and EOS. This suggests that a merger with small value of QM 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 > ∼ 0.01M⊙ irrespective of QM . 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 > ∼ 5 × 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 ≈ 6.5-7 kHz and ∼ 10 −22 at a distance of 50 Mpc for the binary of mass M ≈ 2.7-2.9M⊙.

Sources of Gravitational Waves: Theory and Observations

“…In the case of a binary composed of two NSs the merger dynamics can be quite complex and depends on the binary's total mass M, the mass of the final remnant M f and the maximum NS mass M NS max allowed by the (unknown) NS EoS. For majority of mergers the final remnant is expected to be a BH with or without an accretion disk, except on rare occasions when M f < M NS max , the final remnant can be a NS [303][304][305]. A BH with an accretion disk might promptly form if M NS max < M < ∼ 3 M and the component masses are different from each other.…”

Hybrid approach to black hole perturbations from extended matter sources

“…The torus can be located within a few tens of km from the horizon and could also be subject to a dynamical instability which would induce its accretion onto the black hole on a dynamical timescale (see [17,18] and references therein). High-density tori could be generated in many astrophysical scenarios (see [16,18] for a general overview): in the coalescence of black holeneutron star or neutron star binaries, as shown by several numerical simulations [19] in the collapse to a black hole of a rapidly rotating supramassive neutron star [20,21], as a byproduct of collapsars, that is, massive stars in which iron core-collapse does not produce an explosion, but forms a black hole [22]. As discussed in [18], these systems can be created with event rates comparable to that of corecollapse supernovae.…”

Gravitational Waves from Relativistic Neutron-Star Mergers with Microphysical Equations of State

“…[4] and refs. therein), or have employed a microphysical EoS together with an approximative neutrino treatment while describing gravity in a Newtonian framework (e.g., [5,6]).…”

“…Two T = 0 EoSs, the Shen-EoS [14], and the Lattimer-Swesty-EoS [15], an ideal-gas EoS with parameters chosen to mimic the Shen-EoS, and the APR-EoS [16] were used. The APR-EoS was extended by an ideal-gas-like thermal pressure contribution that is proportional to the internal energy increase due to shock heating and viscous heating [4]. The size of this contribution is determined by an adiabatic index Γ th for which we chose two different values (Γ th = 1.5, 2) in order to investigate its influence on the merger outcome.…”