Neutrino Fast Flavor Instability in three dimensions for a Neutron Star Merger

Abstract

The flavor evolution of neutrinos in core collapse supernovae and neutron star mergers is a critically important unsolved problem in astrophysics. Following the electron flavor evolution of the neutrino system is essential for calculating the thermodynamics of compact objects as well as the chemical elements they produce. Accurately accounting for flavor transformation in these environments is challenging for a number of reasons, including the large number of neutrinos involved, the small spatial scale of the oscillation, and the nonlinearity of the system. We take a step in addressing these issues by presenting a method which describes the neutrino fields in terms of angular moments. Our moment method successfully describes the fast flavor neutrino transformation phenomenon which is expected to occur in regions close to the central object. We apply our moment method to neutron star merger conditions and show that we are able to capture the three phases of growth, saturation, and decoherence by comparing with particle-in-cell calculations. We also determine the size of the growing fluctuations in the neutrino field.

Monte-Carlo Neutrino Transport in Neutron Star Merger Simulations

“…The Matter-Neutrino Resonance (MNR) occurs when the matter potential is equal to the neutrino self-interaction potential, and can impact the luminosity of ν e and νe within a few radii of the post-merger remnant [72,73]. The flast-flavor instability (FFI), on the other hand, is due solely to the neutrino self-potential, and seems to occur in regions where the sign of the net lepton flux (number flux of ν e minus number flux of νe ) changes for different directions of propagation of the neutrinos [74][75][76]. The FFI occurs on very short timescales (∼ns, i.e.…”

“…The FFI occurs on very short timescales (∼ns, i.e. cm length scales), and is likely active in many regions close to the post-merger remnant [76]. How much flavor transformation occurs as a result of the FFI remains uncertain, but recent studies using simplified prescription for where the FFI occurs and how much flavor transformation happens as a result have shown that it could plausibly lead to significant changes in the composition of matter outflows [77,78].…”

Evolution of collisional neutrino flavor instabilities in spherically symmetric supernova models

“…For the fast mode, it converts neutrino flavors in a much shorter length scale ∼ (G F n ν ) −1 ∼ O(1) cm with G F the Fermi constant and n ν the neutrino number density. Studies have shown that fast instabilities generally exist in certain regions near or even inside the neutrinosphere in CCSNe modeled by multi-dimensional simulations [37][38][39][40][41][42][43], and are even more ubiquitously present in the postmerger environments of BNSMs [44][45][46][47][48][49][50][51]. The potential importance and the associated very short length scale of fast mode catalyzes recent surges of local dynamical simulations in tiny boxes [29,[51][52][53][54][55][56][57][58][59][60].…”

Neutrino transport in general relativistic neutron star merger simulations

“…The matter-neutrino resonance (MNR) occurs when the matter potential is equal to the neutrino self-interaction potential, and can impact the luminosity of m e and m e within a few radii of the post-merger remnant (Caballero et al 2014;Zhu et al 2016). The flast-flavor instability (FFI), on the other hand, is due solely to the neutrino self-potential, and seems to occur in regions where the sign of the net lepton flux (number flux of m e minus number flux of m e ) changes between different directions of propagation of the neutrinos (Banerjee et al 2011;Wu et al 2017;Grohs et al 2022). The FFI occurs on very short timescales ( $ ns, i.e., cm length scales), and is likely active in many regions close to the post-merger remnant (Grohs et al 2022).…”

“…The flast-flavor instability (FFI), on the other hand, is due solely to the neutrino self-potential, and seems to occur in regions where the sign of the net lepton flux (number flux of m e minus number flux of m e ) changes between different directions of propagation of the neutrinos (Banerjee et al 2011;Wu et al 2017;Grohs et al 2022). The FFI occurs on very short timescales ( $ ns, i.e., cm length scales), and is likely active in many regions close to the post-merger remnant (Grohs et al 2022). How much flavor transformation occurs as a result of the FFI remains uncertain, but recent studies using simplified prescriptions for where the FFI occurs and how much flavor transformation happens as a result have shown that it could plausibly lead to significant changes in the composition of matter outflows (Li and Siegel 2021;Fernández et al 2022).…”