Far-from-equilibrium bulk-viscous transport coefficients in neutron star mergers

TL;DR

This paper explores the nonlinear bulk-viscous transport properties of neutron star matter in mergers, introducing a new formulation that accounts for far-from-equilibrium effects and depends on nuclear equation of state parameters.

Contribution

It develops a far-from-beta-equilibrium description of bulk viscosity using a new Israel-Stewart formulation with resummed coefficients, accounting for nonlinear effects in neutron star mergers.

Findings

Nonlinear bulk viscosity effects are significant at certain densities.

Transport coefficients depend strongly on nuclear symmetry energy parameters.

Constraints on nuclear parameters influence the modeling of neutron star merger dynamics.

Abstract

We investigate the weak-interaction-driven bulk-viscous transport properties of $npe$ matter in the neutrino transparent regime. Previous works assumed that the induced bulk viscosity correction to pressure, near beta equilibrium, is linear in deviations from the equilibrium charge fraction. We show that this is not always true for (some) realistic equations of state at densities between one and three times saturation density. This nonlinear nature of the perturbation around equilibrium motivates a far-from-beta-equilibrium description of bulk-viscous transport in neutron star mergers, which can be precisely achieved using a new Israel-Stewart formulation with resummed bulk and relaxation time transport coefficients. The computation of these transport coefficients depends on out-of-beta-equilibrium pressure corrections, which can be computed for a given equation of state. We calculate these coefficients for equations of state that satisfy the latest constraints from multi-messenger observations from LIGO/VIRGO and NICER. We show that varying the nuclear symmetry energy $J$ and its slope $L$ can significantly affect the transport coefficients and the nonlinear behavior of the out-of-equilibrium pressure corrections. Therefore, having better constraints on $J$ and $L$ will directly impact our understanding of bulk-viscous processes in neutron star mergers.