Bulk Viscous Damping of Density Oscillations in Neutron Star Mergers

TL;DR

This paper investigates how bulk viscosity from weak interactions dampens density oscillations in neutron star mergers, affecting the post-merger evolution especially at certain temperatures and densities.

Contribution

It models bulk viscous damping in neutron star mergers using relativistic density functional theory, highlighting the temperature-dependent resonance effects.

Findings

Bulk viscosity peaks near neutrino trapping temperature.

Damping timescales can be as short as 10 ms.

Bulk viscosity significantly influences post-merger dynamics at specific conditions.

Abstract

In this paper, we discuss the damping of density oscillations in dense nuclear matter in the temperature range relevant to neutron star mergers. This damping is due to bulk viscosity arising from the weak interaction ``Urca'' processes of neutron decay and electron capture. The nuclear matter is modelled in the relativistic density functional approach. The bulk viscosity reaches a resonant maximum close to the neutrino trapping temperature, then drops rapidly as temperature rises into the range where neutrinos are trapped in neutron stars. We investigate the bulk viscous dissipation timescales in a post-merger object and identify regimes where these timescales are as short as the characteristic timescale $\sim$10 ms, and, therefore, might affect the evolution of the post-merger object. Our analysis indicates that bulk viscous damping would be important at not too high temperatures of the order of a few MeV and densities up to a few times saturation density.