Gravitational Waves from Binary Neutron Star Mergers with a Spectral Equation of State

TL;DR

This paper introduces a spectral equation of state for simulating binary neutron star mergers, balancing physical realism and numerical accuracy, and evaluates its effectiveness in gravitational wave predictions.

Contribution

It develops and tests a new spectral equation of state that offers greater flexibility and smoothness, reducing numerical errors in neutron star merger simulations.

Findings

Spectral equations of state improve simulation accuracy.

Phase errors are minimized with spectral models.

Gravitational wave phase differences are within detectable limits.

Abstract

In numerical simulations of binary neutron star systems, the equation of state of the dense neutron star matter is an important factor in determining both the physical realism and the numerical accuracy of the simulations. Some equations of state used in simulations are $C^2$ or smoother in the pressure/density relationship function, such as a polytropic equation of state, but may not have the flexibility to model stars or remnants of different masses while keeping their radii within known astrophysical constraints. Other equations of state, such as tabular or piece-wise polytropic, may be flexible enough to model additional physics and multiple stars' masses and radii within known constraints, but are not as smooth, resulting in additional numerical error. We will study in this paper a recently developed family of equation of state, using a spectral expansion with sufficient free parameters to allow for a larger flexibility than current polytropic equations of state, and with sufficient smoothness to reduce numerical errors compared to tabulated or piece-wise polytropic equations of state. We perform simulations at three mass ratios with a common chirp mass, using two distinct spectral equations of state, and at multiple numerical resolutions. We evaluate the gravitational waves produced from these simulations, comparing the phase error between resolutions and equations of state, as well as with respect to analytical models. From our simulations we estimate that the phase difference at merger for binaries with a dimensionless weighted tidal deformability difference greater than $\Delta \tilde{\Lambda} = 55$ can be captured by the SpEC code for these equations of state.