The time evolution of fast flavor crossings in post-merger disks around a black hole remnant

TL;DR

This study investigates the time evolution of electron lepton number crossings in neutrino distributions around a black hole post-merger disk, revealing their transient nature and potential impact on nucleosynthesis.

Contribution

It provides a detailed postprocessing analysis of neutrino flavor crossings in a 3D GR simulation, showing their early ubiquity and later confinement near the equator, contrasting with previous persistent crossing reports.

Findings

ELN-XLN crossings are common at ~11 ms post-merger.

Crossings diminish and are localized near the equator at later times.

Implications for heavy element nucleosynthesis in neutron star mergers.

Abstract

We postprocess a three-dimensional general relativistic, full transport neutrino radiation magnetohydrodynamics simulation of the black hole--accretion disk--wind system thought to be a potential outcome of the GW170817 merger to investigate the presence of electron lepton number (ELN-XLN) crossings in the neutrino angular distribution. Neutrinos are evolved with an explicit Monte Carlo method and can interact with matter via emission, absorption, or scattering. Within the postprocessing framework, we find ubiquitous occurrence of ELN-XLN crossings at early times ($\sim$ 11ms) but this does not hold for later times in the simulation. At postmerger times of $ \sim$ 60 ms and beyond, ELN-XLN crossings are only present near the equator. We provide a detailed analysis of the neutrino radiation field to investigate the origin and time evolution of these crossings. Previous reports have suggested ubiquitous flavor crossings persisting throughout the simulation lifetime, albeit for different sets of conditions for the merger remnant, the treatment of hydrodynamics and neutrino transport. Even though we do not perform a direct comparison with other published works, we qualitatively assess the reasons for the difference with our results. The geometric structure and evolution of the ELN-XLN crossings found in our analysis, and by extension, fast flavor instabilities have important implications for heavy element nucleosynthesis in neutron star mergers.