Binary Neutron Star Merger Simulations with a Calibrated Turbulence Model

Abstract

Magnetohydrodynamic (MHD) turbulence in neutron star (NS) merger remnants can impact their evolution and multi-messenger signatures, complicating the interpretation of present and future observations. Due to the high Reynolds numbers and the large computational costs of numerical relativity simulations, resolving all the relevant scales of the turbulence will be impossible for the foreseeable future. Here, we adopt a method to include subgrid-scale turbulence in moderate resolution simulations by extending the large-eddy simulation (LES) method to general relativity (GR). We calibrate our subgrid turbulence model with results from very-high-resolution GRMHD simulations, and we use it to perform NS merger simulations and study the impact of turbulence. We find that turbulence has a quantitative, but not qualitative, impact on the evolution of NS merger remnants, on their gravitational wave signatures, and on the outflows generated in binary NS mergers. Our approach provides a viable path to quantify uncertainties due to turbulence in NS mergers.

Neutron star merger remnants

“…The magnitudes of these effects depends on the particular value assumed for the α-viscosity subgrid model. For example, the use of a turbulence model calibrated to the high-resolution MHD runs of [152], leads to significant changes to the subdominant features of the GW spectrum and to the ejecta [158]. However, neutrino effects on the ejecta are comparatively more relevant than magnetohydrodynamical turbulence.…”

General relativistic MHD large eddy simulations with gradient subgrid-scale model

“…In an explicit LES approach, the evolution equations for the resolved fields are supplemented with explicit subgridscale (SGS) terms which effectively model the small scales [19]. So far, to the best of our knowledge, only a couple of works have introduced a SGS model into a GR simulation [20,21]. Radice took into account the dissipative effects of magnetic fields on the postmerger differentially rotating fluid by implementing a purely viscous SGS model [22], originally designed for incompressible hydrodynamics.…”

Turbulent magnetic-field amplification in the first 10 milliseconds after a binary neutron star merger: Comparing high-resolution and large-eddy simulations

“…Other approaches have, instead, centered their attention on the turbulent viscous effect during the postmerger phase, evolving viscous hydrodynamics (HD) in substitution of the MHD equations [36][37][38][39]. These models are however unable, by construction, to capture the dynamo mechanism and depend on parameters to be calibrated via very high resolution GRMHD simulations (e.g., [40]).…”