PREPRINT
3F42E6E3-1A9F-40C7-937D-F4C7C7C60BB7

# Muons in the aftermath of Neutron Star Mergers and their impact on Trapped Neutrinos

Eleonora Loffredo, Albino Perego, Domenico Logoteta, Marica Branchesi

Submitted on 9 September 2022

## 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 thermodynamics conditions favor their production. Our work aims at investigating the impact of muons on the merger remnant. We post-process the outcome of four numerical relativity simulations, performed with three different baryonic EOSs and two mass ratios, considering the first $15$ milliseconds after the 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 the net fraction of muons is between $30\mathrm{%}$ and $70\mathrm{%}$ the one of electrons, depending on the baryonic EOS. Muons change the flavour hierarchy of trapped (anti)neutrinos, so that muon anti-neutrinos are the most abundant, followed by electron anti-neutrinos. Finally, muons modify the neutron to proton ratio inducing variations of the remnant pressure up to $7\mathrm{%}$. This work demonstrates that muons have a non-negligible effect on the outcome of BNS merger simulations, and they should be included to improve simulations accuracy.

## Preprint

Comment: 16 pages, 11 figures, submitted to A&A

Subject: Astrophysics - High Energy Astrophysical Phenomena