Bulk Viscosity of Relativistic $npe\mu$ Matter in Neutron-Star Mergers

Abstract

We discuss the bulk viscosity of hot and dense $npe\mu$ matter arising from weak-interaction direct Urca processes. We consider two regimes of interest: (a) the neutrino-transparent regime with $T\le T_{\mathrm{tr}}$ ($T_{\mathrm{tr}}\simeq 5÷10$ MeV is the neutrino-trapping temperature); and (b) the neutrino-trapped regime with $T\ge T_{\mathrm{tr}}$. Nuclear matter is modeled in relativistic density functional approach with density-dependent parametrization DDME2. The maximum of the bulk viscosity is achieved at temperatures $T\simeq 5÷6$ MeV in the neutrino-transparent regime, then it drops rapidly at higher temperatures where neutrino-trapping occurs. As an astrophysical application, we estimate the damping timescales of density oscillations by the bulk viscosity in neutron star mergers and find that, e.g., at the oscillation frequency $f=10$ kHz, the damping will be very efficient at temperatures $4\le T\le 7$ MeV where the bulk viscosity might affect the evolution of the post-merger object.

16 pages, 9 figures, matches published version

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• Numerical modelling of bulk viscosity in neutron stars
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• Tabulated Equations of State From Models Informed by Chiral Effective Field Theory
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• Gravitational Wave emission in Binary Neutron Star early post-merger within a dark environment
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• Tidal dissipation in binary neutron star inspirals from hyperon bulk viscosity: Phase modeling and parameter estimation bias
• Neutron star evolution with the Bemfica-Disconzi-Noronha-Kovtun viscous hydrodynamics framework
• Thermal and Magnetic effects on Bulk Viscosity in Binary Neutron Star Mergers