We clarify how the weak-interaction-driven bulk viscosity $\zeta$ and the bulk relaxation time ${\tau }_{\mathrm{\Pi }}$ of neutrino-transparent $npe$ matter depend on the nuclear symmetry energy. We show that, at saturation density, the equation-of-state dependence of these transport quantities is fully determined by the experimentally constrained nuclear symmetry energy $S$ and its slope $L$. Variations of $L$ can change the bulk viscosity by orders of magnitude, which can affect both the dissipative and the conservative tidal response of neutron stars. This suggests that both conservative and dissipative effects encoded in the gravitational-wave signatures of binary neutron star inspirals may help constrain nuclear symmetry-energy properties.

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• Nuclear astrophysics
• Nuclear matter in neutron stars
• Neutron stars & pulsars