On the importance of viscous dissipation and heat conduction in binary neutron-star mergers

TL;DR

This paper investigates the roles of viscous dissipation and heat conduction in neutron star mergers, highlighting conditions under which these effects influence post-merger dynamics and gravitational wave signals.

Contribution

It provides a detailed analysis of dissipation timescales and identifies bulk viscosity as potentially significant in neutron star merger simulations.

Findings

Thermal transport and shear viscosity are negligible unless neutrino trapping occurs.

Bulk viscous dissipation can significantly damp density oscillations post-merger.

Bulk viscosity is near its resonant maximum in typical neutron-star mergers.

Abstract

Inferring the properties of dense matter is one of the most exciting prospects from the measurement of gravitational waves from neutron star mergers. However, it will require reliable numerical simulations that incorporate viscous dissipation and energy transport if these can play a significant role within the survival time of the post-merger object. We calculate timescales for typical forms of dissipation and find that thermal transport and shear viscosity will not be important unless neutrino trapping occurs, which requires temperatures above about 10 MeV and gradients over lengthscales of 0.1 km or less. On the other hand, if direct-Urca processes remain suppressed, leaving modified-Urca processes to establish flavor equilibrium, then bulk viscous dissipation could provide significant damping to density oscillations observed right after the merger. When comparing with data from a state-of-the-art merger simulation we find that the bulk viscosity takes values close to its resonant maximum in a typical neutron-star merger, motivating a more careful assessment of the role of bulk viscous dissipation in the gravitational-wave signal from merging neutron stars.