The Astrophysical Journal Letters (Jan 2024)

Emergence of Microphysical Bulk Viscosity in Binary Neutron Star Postmerger Dynamics

  • Elias R. Most,
  • Alexander Haber,
  • Steven P. Harris,
  • Ziyuan Zhang,
  • Mark G. Alford,
  • Jorge Noronha

DOI
https://doi.org/10.3847/2041-8213/ad454f
Journal volume & issue
Vol. 967, no. 1
p. L14

Abstract

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In nuclear matter in isolated neutron stars, the flavor content (e.g., proton fraction) is subject to weak interactions, establishing flavor ( β -)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 postmerger simulations can emerge and assess the bulk viscous dynamics of the resulting flow. We further place limits on the impact of the postmerger gravitational-wave strain. Our results show that weak-interaction-driven bulk viscosity can potentially lead to a phase shift of the postmerger gravitational-wave spectrum, although the effect is currently on the same level as the numerical errors of our simulation.

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