Quasiequilibrium sequences of synchronized and irrotational binary neutron stars in general relativity. I. Method and tests

TL;DR

This paper introduces a numerical method for computing quasiequilibrium configurations of close binary neutron stars, focusing on both synchronized and irrotational states, using an approximation of general relativity and spectral methods.

Contribution

The paper develops a multi-domain spectral method to accurately model binary neutron stars in quasiequilibrium, including the complex irrotational case, with extensive numerical tests and evolutionary sequences.

Findings

High-precision models of binary neutron stars in quasiequilibrium.

Effective treatment of irrotational and synchronized states.

Validated numerical code with analytical comparisons.

Abstract

We present a numerical method to compute quasiequilibrium configurations of close binary neutron stars in the pre-coalescing stage. A hydrodynamical treatment is performed under the assumption that the flow is either rigidly rotating or irrotational. The latter state is technically more complicated to treat than the former one (synchronized binary), but is expected to represent fairly well the late evolutionary stages of a binary neutron star system. As regards the gravitational field, an approximation of general relativity is used, which amounts to solving five of the ten Einstein equations (conformally flat spatial metric). The obtained system of partial differential equations is solved by means of a multi-domain spectral method. Two spherical coordinate systems are introduced, one centered on each star; this results in a precise description of the stellar interiors. Thanks to the multi-domain approach, this high precision is extended to the strong field regions. The computational domain covers the whole space so that exact boundary conditions are set to infinity. Extensive tests of the numerical code are performed, including comparisons with recent analytical solutions. Finally a constant baryon number sequence (evolutionary sequence) is presented in details for a polytropic equation of state with gamma=2.