Neutron dark decays modify the equation of state and either mildly suppress or strongly enhance bulk viscosity in neutron star merger conditions, depending on the in-medium decay rate.
Large amplitude behavior of the bulk viscosity of dense matter
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abstract
We study the bulk viscosity of dense matter, taking into account non-linear effects which arise in the large amplitude "supra-thermal" region where the deviation $\mu_\Delta$ of the chemical potentials from chemical equilibrium fulfills $\mu_\Delta>T$. This regime is relevant to unstable modes such as r-modes, which grow in amplitude until saturated by non-linear effects. We study the damping due to direct and modified Urca processes in hadronic matter, and due to nonleptonic weak interactions in strange quark matter. We give general results valid for an arbitrary equation of state of dense matter and find that the viscosity can be strongly enhanced by supra-thermal effects. Our study confirms previous results on quark matter and shows that the non-linear enhancement is even stronger in the case of hadronic matter. Our results can be applied to calculations of the r-mode-induced spin-down of fast-rotating neutron stars, where the spin-down time will depend on the saturation amplitude of the r-mode
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Bayesian inference on observational data yields shear viscosity timescale τ_s=(4.99^{+0.49}_{-0.52})×10^8 T^{5/3} s and bulk viscosity timescale for two-layer hybrid stars, giving frequency minima of 451.87 Hz and 517.47 Hz that explain stability of pulsars including XTE J0929-314.
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Bulk viscosity from neutron decays to dark baryons in neutron star matter
Neutron dark decays modify the equation of state and either mildly suppress or strongly enhance bulk viscosity in neutron star merger conditions, depending on the in-medium decay rate.
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Modelling Dissipative Dynamics of r-mode Instability in Hybrid Stars
Bayesian inference on observational data yields shear viscosity timescale τ_s=(4.99^{+0.49}_{-0.52})×10^8 T^{5/3} s and bulk viscosity timescale for two-layer hybrid stars, giving frequency minima of 451.87 Hz and 517.47 Hz that explain stability of pulsars including XTE J0929-314.