Simulating relativistic, dissipative hydrodynamics for neutron star mergers

Abstract

Neutron stars are astrophysical compact objects that contain a host of extreme physics including strong gravity, immense magnetic fields and highly dense nuclear matter. Neutron star mergers are violent enough to probe this physics in a meaningful way through our detection of the electromagnetic and gravitational waves we receive from these events.

However, their complexity necessitates the use of numerical simulations to evolve the curved spacetime, as well as the fluid used to describe the neutron stars themselves. Most simulations to date use an ‘ideal fluid’ description - a simplification that reduces computational cost but misses out-of-equilibrium effects like dissipation.

Here, we compare results from a number of both established and emerging models of non-ideal hydrodynamics. This includes results from our novel formulation of dissipative hydrodynamics, which we show is able to capture viscosity and heat conductivity near the ideal fluid limit in a highly efficient computational way.

Even the highest resolution simulations of binary neutron star mergers are unable to resolve all the dynamical scales of these systems, leaving unresolved, subgrid microphysics. This can produce macroscale effects, which our field of research has recently begun to model using explicit large-eddy simulations.

To advance this direction, we also present the first fully-covariant, Lagrangian filtering scheme applied to relativistic turbulence. We show that a dissipative fluid prescription, with statistically-fitted non-ideal parameters, may be used as a closure scheme to describe the residuals introduced by explicitly filtering fine-scale fluid flow.

These pieces of work, together, will enable us to perform higher resolution simulations of neutron star mergers that include more of the relevant physics. This will serve us well when trying to obtain accurate gravitational waveforms and electromagnetic signals to compare against future detections from next-generation detectors.

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Identifiers

ORCID for Nils Andersson: orcid.org/0000-0001-8550-3843

ORCID for Ian Hawke: orcid.org/0000-0003-4805-0309

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