Binary neutron stars with generic spin, eccentricity, mass ratio, and compactness: Quasi-equilibrium sequences and first evolutions

Information about the last stages of a binary neutron star inspiral and the final merger can be extracted from quasiequilibrium configurations and dynamical evolutions. In this article, we construct quasiequilibrium configurations for different spins, eccentricities, mass ratios, compactnesses, and equations of state. For this purpose we employ the sgrid code, which allows us to construct such data in previously inaccessible regions of the parameter space. In particular, we consider spinning neutron stars in isolation and in binary systems; we incorporate new methods to produce highly eccentric and eccentricity-reduced data; we present the possibility of computing data for significantly unequal-mass binaries with mass ratios q≃2; and we create equal-mass binaries with individual compactness up to C≃0.23. As a proof of principle, we explore the dynamical evolution of three new configurations. First, we simulate a q=2.06 mass ratio which is the highest mass ratio for a binary neutron star evolved in numerical relativity to date. We find that mass transfer from the companion star sets in a few revolutions before merger and a rest mass of ∼10-2M is transferred between the two stars. This amount of mass accretion corresponds to ∼1051 ergs of accretion energy. This configuration also ejects a large amount of material during merger (∼7.6×10-2M), imparting a substantial kick to the remnant neutron star. Second, we simulate the first merger of a precessing binary neutron star. We present the dominant modes of the gravitational waves for the precessing simulation, where a clear imprint of the precession is visible in the (2,1) mode. Finally, we quantify the effect of an eccentricity-reduction procedure on the gravitational waveform. The procedure improves the waveform quality and should be employed in future precision studies. However, one also needs to reduce other errors in the waveforms, notably truncation errors, in order for the improvement due to eccentricity reduction to be effective.