Merger of binary neutron stars of unequal mass in full general relativity

Abstract

We present results of three dimensional numerical simulations of the merger of unequal-mass binary neutron stars in full general relativity. A ⌫-law equation of state Pϭ(⌫Ϫ1) is adopted, where P, , , and ⌫ are the pressure, rest mass density, specific internal energy, and the adiabatic constant, respectively. We take ⌫ϭ2 and the baryon rest-mass ratio Q M to be in the range 0.85-1. The typical grid size is (633,633,317) for (x,y,z). We improve several implementations since the latest work. In the present code, the radiation reaction of gravitational waves is taken into account with a good accuracy. This fact enables us to follow the coalescence all the way from the late inspiral phase through the merger phase for which the transition is triggered by the radiation reaction. It is found that if the total rest mass of the system is more than ϳ1.7 times of the maximum allowed rest mass of spherical neutron stars, a black hole is formed after the merger, irrespective of the mass ratios. The gravitational waveforms and outcomes in the merger of unequal-mass binaries are compared with those in equal-mass binaries. It is found that the disk mass around the so formed black holes increases with decreasing rest-mass ratios and decreases with increasing compactness of neutron stars. The merger process and the gravitational waveforms also depend strongly on the rest-mass ratios even for the range Q M ϭ0.85-1.

Relativistic outflows from remnants of compact object mergers and their viability for short gamma-ray bursts

“…A high density halo would have to be expected if, for example, the collapse to a black hole were delayed due to the effects of very rapid (differential) rotation or viscous heating (e.g., Duez et al 2004;Morrison et al 2004), in which case the hot neutron star would radiate neutrinos and a neutrino-driven wind (e.g., Duncan et al 1986) would lead to a dense, expanding baryonic cloud around the merger site. It may also be possible to find situations where the accretion torus is surrounded by a thin, dilute halo, in particular, if the BH forms during the merger or within a few dynamical timescales afterwards (e.g., Shibata & Uryū 2000;Shibata et al 2003;Oechslin et al 2004). In regard of these considerations we have performed two series of simulations.…”

Magnetohydrodynamics in full general relativity: Formulation and tests

“…Our formulation for Einstein's equations is the same as in [6] in three spatial dimensions and in [34] in axial symmetry. Here, we briefly review the basic equations in our formulation.…”

“…In the past several years, this field has been extensively developed (e.g., [1][2][3][4][5][6][7]) and, as a result, now it is feasible to perform accurate simulations of such general relativistic phenomena for yielding scientific results (e.g., [6][7][8][9] for our latest results). For example, with the current implementation, radiation reaction of gravitational waves in the merger of binary neutron stars can be taken into account within ∼ 1% error in an appropriate computational setting [6,7]. This fact illustrates that the numerical relativity is a robust approach for detailed theoretical study of astrophysical phenomena and gravitational waves emitted.…”

“…This implies that a small seed field can wind up and grow in the complex motion of the matter, resulting in a significant effect in the dynamics of the matter such as magnetically driven wind or jet and angular momentum redistribution. Specifically, in the context of the general relativistic astrophysics, the magnetic fields will play a role in the following phenomena and objects: Stellar core collapse of magnetized massive stars to a protoneutron star [11] or a black hole, stability of accretion disks (which are either non-self-gravitating or self-gravitating) around black holes and neutron stars, magnetic braking of differentially rotating neutron stars [10] which are formed after merger of binary neutron stars [6,7] and stellar core collapse [14][15][16]8,9], and magnetically induced jet around the compact objects (e.g., [17]). To clarify these phenomena, fully general relativistic MHD (GRMHD) simulation (involving dynamical spacetimes) is probably the best theoretical approach.…”

Sources of Gravitational Waves: Theory and Observations

“…This phase could wit-ness quite a lot of rotational energy emitted as GWs, with a spectrum that is characteristic of the NS EoS [308,309]. A BH without an accretion disk is not a very likely outcome, but it can happen if M > ∼ 3 M [303, 304,310]. The hypermassive NS phase and BHs with accretion disks could both be accompanied by significant emission of electromagnetic radiation [311].…”