Projecting the likely importance of weak-interaction-driven bulk viscosity in neutron star mergers

TL;DR

This paper estimates the potential impact of weak-interaction-driven bulk viscosity on gravitational wave signals from neutron star mergers, finding it likely negligible during inspiral but possibly significant post-merger, especially in different scenarios.

Contribution

It provides the first assessment of bulk viscosity effects on neutron star merger gravitational waves, highlighting potential significance in post-merger dynamics based on simulation data.

Findings

Bulk viscosity affects binding energy at 4PN order during inspiral.

Bulk viscosity could significantly influence post-merger dynamics.

Effects may be larger in different merger scenarios or with non-linear effects.

Abstract

In this work, we estimate how much bulk viscosity driven by Urca processes is likely to affect the gravitational wave signal of a neutron star coalescence. In the late inspiral, we show that bulk viscosity affects the binding energy at fourth post-Newtonian (PN) order. Even though this effect is enhanced by the square of the gravitational compactness, the coefficient of bulk viscosity is likely too small to lead to observable effects in the waveform during the late inspiral, when only considering the orbital motion itself. In the post-merger, however, the characteristic time-scales and spatial scales are different, potentially leading to the opposite conclusion. We post-process data from a state-of-the-art equal-mass binary neutron star merger simulation to estimate the effects of bulk viscosity (which was not included in the simulation itself). In that scenario, we find that bulk viscosity can reach high values in regions of the merger. We compute several estimates of how much it might directly affect the global dynamics of the considered merger scenario, and find that it could become significant. Even larger effects could arise in different merger scenarios or in simulations that include non-linear effects. This assessment is reinforced by a quantitative comparison with relativistic heavy-ion collisions where such effects have been explored extensively.