Large amplitude behavior of the bulk viscosity of dense matter

TL;DR

This paper investigates the non-linear behavior of bulk viscosity in dense matter, especially at large amplitudes relevant to neutron star r-modes, revealing significant enhancement effects that influence stellar spin-down.

Contribution

It provides a general analysis of bulk viscosity in dense matter considering non-linear effects, applicable to various equations of state and relevant to neutron star oscillation damping.

Findings

Bulk viscosity can be significantly enhanced by supra-thermal effects.

Non-linear effects are stronger in hadronic matter than in quark matter.

Results impact calculations of neutron star spin-down due to r-modes.

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