Emergence of microphysical bulk viscosity in binary neutron star post-merger dynamics

Understand

In nuclear matter in isolated neutron stars, the flavor content (e.g., proton fraction) is subject to weak interactions, establishing flavor ($\beta$-)equilibrium.

• However, there can be deviations from this equilibrium during the merger of two neutron stars.
• We study the resulting out-of-equilibrium dynamics during the collision by incorporating direct and modified Urca processes (in the neutrino-transparent regime) into general-relativistic hydrodynamics simulations with a simplified neutrino transport scheme.
• We demonstrate how weak-interaction-driven bulk viscosity in post-merger simulations can emerge and assess the bulk viscous dynamics of the resulting flow.