Thinking Outside the Box: Numerical Relativity with Particles

The observation of gravitational waves from compact objects has now become an active part of observational astronomy. For a sound interpretation, one needs to compare such observations against detailed Numerical Relativity simulations, which are essential tools to explore the dynamics and physics of compact binary mergers. To date, essentially all simulation codes that solve the full set of Einstein's equations are performed in the framework of Eulerian hydrodynamics. The exception is our recently developed Numerical Relativity code SPHINCS_BSSN which solves the commonly used BSSN formulation of the Einstein equations on a structured mesh and the matter equations via Lagrangian particles. We show here, for the first time, SPHINCS_BSSN neutron star merger simulations with piecewise polytropic approximations to four nuclear matter equations of state. In this set of neutron star merger simulations, we focus on perfectly symmetric binary systems that are irrotational and have 1.3 M-circle dot masses. We introduce some further methodological refinements (a new way of steering dissipation, an improved particle-mesh mapping), and we explore the impact of the exponent that enters in the calculation of the thermal pressure contribution. We find that it leaves a noticeable imprint on the gravitational wave amplitude (calculated via both quadrupole approximation and the Psi(4) formalism) and has a noticeable impact on the amount of dynamic ejecta. Consistent with earlier findings, we only find a few times 10-3M(circle dot) as dynamic ejecta in the studied equal mass binary systems, with softer equations of state (which are more prone to shock formation) ejecting larger amounts of matter. In all of the cases, we see a credible high-velocity (similar to 0.5...0.7c) ejecta component of similar to 10-4M(circle dot) that is launched at contact from the interface between the two neutron stars. Such a high-velocity component has been suggested to produce an early, blue precursor to the main kilonova emission, and it could also potentially cause a kilonova afterglow.

Automated summary

Gravitational wave observation has become an important part of astronomy, and numerical relativity simulations are essential for interpreting such observations. Our recently developed numerical relativity code SPHINCS_BSSN is capable of simulating neutron star mergers and has shown noticeable effects of varying exponents on gravitational wave amplitude and dynamic ejecta. High-velocity ejecta components have been observed, suggesting their potential role in early precursor and afterglow of kilonova emission.