Muons in the aftermath of neutron star mergers and their impact on trapped neutrinos

TL;DR

This paper investigates the role of muons in neutron star merger remnants, showing they significantly influence neutrino composition and pressure, thus impacting the accuracy of merger simulations.

Contribution

It is the first study to analyze the impact of muons on BNS merger remnants using post-processed numerical relativity simulations.

Findings

Muons constitute 30-70% of electrons depending on the EOS.

Muons alter the neutrino flavor hierarchy inside the remnant.

They modify the neutron-to-proton ratio and pressure by up to 7%.

Abstract

In the upcoming years, present and next-generation gravitational wave observatories will detect a larger number of binary neutron star (BNS) mergers with increasing accuracy. In this context, improving BNS merger numerical simulations is crucial to correctly interpret the data and constrain the equation of state (EOS) of neutron stars (NSs). State-of-the-art simulations of BNS mergers do not include muons. However, muons are known to be relevant in the microphysics of cold NSs and are expected to have a significant role in mergers, where the typical thermodynamic conditions favour their production. Our work is aimed at investigating the impact of muons on the merger remnant. We post-process the outcome of four numerical relativity simulations of BNS mergers performed with three different baryonic EOSs and two mass ratios considering the first $15$ milliseconds after merger. We compute the abundance of muons in the remnant and analyse how muons affect the trapped neutrino component and the fluid pressure. We find that depending on the baryonic EOS, the net fraction of muons is between $30 \%$ and $70 \%$ the net fraction of electrons. Muons change the flavour hierarchy of trapped (anti-)neutrinos such that deep inside the remnant, muon anti-neutrinos are the most abundant, followed by electron anti-neutrinos. Finally, muons and trapped neutrinos modify the neutron-to-proton ratio, affecting the remnant pressure by up to $7\%$ when compared with calculations neglecting them. This work demonstrates that muons have a non-negligible effect on the outcome of BNS merger simulations, and they should be included to improve the accuracy of a simulation.