Emergence of Microphysical Bulk Viscosity in Binary Neutron Star Postmerger Dynamics

Creators: Most, Elias R.¹; Haber, Alexander; Harris, Steven P.; Zhang, Ziyuan; Alford, Mark G.; Noronha, Jorge

1. California Institute of Technology

Abstract

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.

Copyright and License

© 2024. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

E.R.M. thanks F. Foucart, J. Noronha-Hostler, A. Pandya, F. Pretorius, C. Raithel, and J. Ripley for insightful discussions related to this work. All authors are grateful to M. Antonelli, S. Bernuzzi, G. Camelio, L. Gavassino, and B. Haskell for very helpful comments on the manuscript and L. Gavassino for pointing out the question of hydrodynamic frame choice associated with Equation (4). E.R.M. acknowledges support for compute time allocations on the NSF Frontera supercomputer under grant AST21006. This work used the Extreme Science and Engineering Discovery Environment (XSEDE; Towns et al. 2014) through Expanse at SDSC and Bridges-2 at PSC through allocations PHY210053 and PHY210074. This work also used Delta at the National Center for Supercomputing Applications (NCSA) through allocation PHY210074 from the Advanced Cyber infrastructure Coordination Ecosystem: Services & Support (ACCESS) program, which is supported by National Science Foundation grants #2138259, #2138286, #2138307, #2137603, and #2138296. The simulations were also in part performed on computational resources managed and supported by Princeton Research Computing, a consortium of groups including the Princeton Institute for Computational Science and Engineering (PICSciE) and the Office of Information Technology's High Performance Computing Center and Visualization Laboratory at Princeton University. E.R.M. acknowledges partial support from the National Science Foundation through PHY-2309210. M.G.A., A.H., and Z.Z. are partly supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under award No. DE-FG02-05ER41375. S.P.H. is supported by U.S. Department of Energy grant DE-FG02-00ER41132 as well as National Science Foundation grant No.PHY-1430152 (JINA Center for the Evolution of the Elements). J.N. is partially supported by the U.S. Department of Energy, Office of Science, Office for Nuclear Physics, under award No. DE-SC0023861. Z.Z. was supported in part by the National Science Foundation (NSF) within the framework of the MUSES collaboration, under grant No. OAC-2103680.

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