Emergence of Microphysical Bulk Viscosity in Binary Neutron Star Postmerger Dynamics

• Published in: Astrophysical journal. Letters Vol. 967; no. 1; p. L14
• Main Authors: Most, Elias R., Haber, Alexander, Harris, Steven P., Zhang, Ziyuan, Alford, Mark G., Noronha, Jorge
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 01.05.2024; IOP Publishing; American Astronomical Society
• Subjects: Binary stars; Collision dynamics; Flavors; Gravitational waves; Hydrodynamics; Neutrinos; Neutron stars; Nuclear astrophysics; Nuclear matter; Nuclear physics; Star mergers; Viscosity; Wave spectra
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ad454f

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.