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

Abstract

Context. 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). Aims. 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. Methods. 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. Results. 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. Conclusions. 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.

General-relativistic Radiation Transport Scheme in Gmunu. II. Implementation of Novel Microphysical Library for Neutrino Radiation—Weakhub

“…The third thermodynamic point (III) corresponds to the hot ring region near the core of a hypermassive neutron star of a BNS merger system (Loffredo et al 2023), and an extremely small fraction of muons is assumed. We computed this point by using the same EOS as point (II).…”

Collective Neutrino Oscillations and Heavy-element Nucleosynthesis in Supernovae: Exploring Potential Effects of Many-body Neutrino Correlations

“…More recently, the role of muons in supernova and merger environments has received greater consideration (e.g.,Bollig et al 2017;Fischer et al 2020a;Guo et al 2020;Loffredo et al 2023). This aspect would necessitate differentiating between μ and τ flavor neutrinos, adding yet another wrinkle to the potential impact of flavor oscillations on the dynamics and nucleosynthesis.…”

Dissipative and shearing dynamics in astrophysical compact objects

“…The microphysical origin of bulk viscosity in BNS mergers is the violation of weak chemical equilibrium (β-equilibrium) through the violent collision process [12,11,141]. Specifically, assuming that cold neutron stars are made up of npe-matter, i.e., they consist only of neutrons, protons and electrons, and neglecting the appearance of muons for simplicity, see also [179,16], different conditions for chemical equilibrium exist which are based on the weak chemical reactions among the constituents of the neutron star matter. Following [141] these reactions are β-processes under various thermodynamic conditions which can be broadly grouped into electron-capture reactions and neutron-decay reactions.…”