Neutrino flavor instabilities in neutron star mergers with moment transport: Slow, fast, and collisional modes

TL;DR

This paper analyzes neutrino flavor instabilities in neutron star mergers using a moment transport framework, revealing the importance of anisotropic effects and different instability modes for accurate simulation predictions.

Contribution

It introduces a linear stability analysis in a multi-energy setting for neutron star mergers, highlighting the interplay of collisional, fast, and slow neutrino flavor instabilities.

Findings

Fast instability dominates most regions.

Some areas exhibit only collisional instability.

Slow instability is significant at large distances and is underestimated without anisotropic effects.

Abstract

Determining where, when, and how neutrino flavor oscillations must be included in large-scale simulations of hot and dense astrophysical environments is an enduring challenge that must be tackled to obtain accurate predictions. Using an angular moment-based linear stability analysis framework, we examine the different kinds of flavor instabilities that can take place in the context of the post-processing of a neutron star merger simulation, with a particular focus on the collisional flavor instability and a careful assessment of several commonly used approximations. First, neglecting anisotropies of the neutrino field, we investigate the extent to which commonly used monoenergetic growth rates reproduce the results obtained from a full multi-energy treatment. Contrary to the large discrepancies found in core-collapse supernova environments, we propose a simple combination of energy-averaged estimates that reproduces the multi-energy growth rates in our representative simulation snapshot. We then quantify the impact of additional physical effects, including nuclear many-body corrections, scattering opacities, and the inclusion of the vacuum term in the neutrino Hamiltonian. Finally, we include the neutrino distribution anisotropies, which allows us to explore, for the first time in a multi-energy setting, the interplay between collisional, fast, and slow modes in a moment-based neutron star merger simulation. We find that, despite a dominance of the fast instability in most of the simulation volume, certain regions exhibit only a collisional instability, while others, especially at large distances, exhibit a slow instability that is largely underestimated if anisotropic effects are neglected.