Speaker
Description
Binary neutron star and neutron star-black hole mergers are established sites for rapid neutron capture (r-process) nucleosynthesis and are associated with short gamma-ray bursts (SGRBs). However, the role of nuclear reactions in powering these transients and the impact of neutron degeneracy on early merger dynamics remain poorly understood. This work synthesizes two complementary studies that collectively demonstrate the critical importance of nuclear physics in merger environments.
First, we show that neutron degeneracy fundamentally alters the early stages of neutron star-black hole mergers. Prior models universally assumed Maxwell-Boltzmann statistics for neutron capture, but neutrons in merger ejecta exist in a highly degenerate state. We developed a modified two-dimensional Saha equation incorporating neutron degeneracy and implemented tabulated degenerate neutron capture rates into the WinNet nuclear reaction network. Our results reveal a two-phase thermal evolution: rapid initial cooling reaching ~10^55 erg s^-1, followed by enhanced late-time heating up to 10^54 erg s^-1 driven by neutron captures, beta decay, and neutron emissions. Degeneracy effects persist for nearly one second, significantly modifying initial nuclear abundances and r-process pathways, though final abundance patterns remain largely unchanged.
Second, we demonstrate that nuclear reactions in the dynamical ejecta of binary mergers generate sufficient energy to contribute to—or potentially power—SGRBs. Across multiple merger trajectories, beta decay contributes ~10^50 erg s^-1, while neutron capture produces extreme, highly variable energy spikes reaching up to 10^58 erg s^-1. The spasmodic nature of neutron capture energy release naturally explains the rapid luminosity variability characteristic of SGRB light curves, challenging the traditional paradigm that SGRBs are solely magnetically powered.
Together, these findings establish that nuclear physics—particularly neutron degeneracy corrections and the energetic contributions of beta decay and neutron capture—plays an essential role in shaping both the nucleosynthetic outcomes and observational signatures of compact binary mergers. We conclude that nuclear reactions are not merely a supplementary effect but constitute a primary driver of early merger phenomenology, with significant implications for kilonova light curves, SGRB emission mechanisms, and the interpretation of multimessenger observations.