ADVANCES IN GENERAL RELATIVISTIC SIMULATIONS OF NEUTRON STARS
Abstract
Increasingly improving gravitational wave (GW) detectors with progressively lower signal-to-noise ratios will provide GW signal data with higher accuracy. They will also detect GW signals from compact body coalescence (CBC) events, such as binary neutron star (BNS) mergers, more frequently. This, in turn, poses a two-fold challenge to the numerical relativity community: generating numerical relativity simulations with greater accuracy at greater speed and performing tailored simulations to investigate observed GW events post-fact. Towards addressing the first challenge, we present here a new hybrid spatial differentiation scheme that combines a discontinuous Galerkin (DG) method highly efficient at handling smooth fields with finite volume (FV) and finite difference (FD) methods that handle non-smooth fields robustly. When parallelizing, the computational mesh is usually divided into smaller elements distributed among the various computational processes, and traditional FV methods require information from up to 26 neighboring elements when using noncuboid patches. This reduces scalability when using large supercomputers. Our FV implementation is compactified to require information exchange only from the surface of 6 neighboring elements that touch but do not overlap. Through this, we plan to retain the DG method’s high scalability when parallelizing the hybrid scheme. We utilize our new method to simulate single neutron star cases, including the challenging cases of a neutron star migrating from an unstable equilibrium to a stable configuration and a boosted neutron star, which are performed for the first time in 3D with full general relativistic hydrodynamics using DG methods. We use the Nmesh program for this purpose. However, Nmesh is a fledgling program still undergoing testing before being deemed fit for BNS simulations. As such, for the second goal of investigating GW events, we use the well-established BAM program. As a case study, we investigate the GW190425 event. Owing to the larger chirp mass and lack of an electromagnetic counterpart, most studies modeled this event as a black hole neutron star event. We, however, simulate BNS systems with mass ratios q = 0.7, 0.8, 0.9, and 1 with the MPA1 equation of state to investigate it. We study the related GW signals and matter behavior.