Accurate evolutions of unequal-mass neutron-star binaries: properties of the torus and short GRB engines

We present new results from accurate and fully general-relativistic simulations of the coalescence of unmagnetized binary neutron stars with various mass ratios. The evolution of the stars is followed through the inspiral phase, the merger, and the prompt collapse to a black hole, up until the appearance of a thick accretion disc, which is studied as it enters and remains in a regime of quasi-steady accretion. Although a simple ideal-fluid equation of state with Γ = 2 is used, this work presents a systematic study within a fully general-relativistic framework of the properties of the resulting black-hole-torus system produced by the merger of unequal-mass binaries. More specifically, we show that (1) the mass of the torus increases considerably with the mass asymmetry, and equal-mass binaries do not produce significant tori if they have a total baryonic mass Mtot ∼> 3.7 Mo-dot; (2) tori with masses Mtor ∼ 0.2 Mo-dot are measured for binaries with Mtot ∼ 3.4 Mo-dot and mass ratios q ∼ 0.75-0.85; (3) the mass of the torus can be estimated by the simple expression Mtor-bar(q,Mtot = [c1 (1-q) + c2](Mmax-Mot), involving the maximum mass for the binaries and coefficients constrained from the simulations, and suggesting that the tori can have masses as large as Mtor-bar ∼ 0.35Modot for Mtot ∼ 2.8 Mo-dot and q ∼ 0.75-0.85; (4) using a novel technique to analyze the evolution of the tori, we find no evidence for the onset of non-axisymmetric instabilities and that very little, if any, of their mass is unbound; (5) finally, for all the binaries considered, we compute the complete gravitational waveforms and the recoils imparted to the black holes, discussing the prospects of the detection of these sources for a number of present and future detectors.