Degenerate Neutron Capture Rates in the Threshold of Neutron Star–Black Hole Mergers

Abstract We first show that the rate of energy generation due to neutron capture rates can be important in neutron star–black hole mergers (NS–BH mergers) by considering neutron degeneracy. Neutrons during NS–BH mergers exist in a highly degenerate state, yet prior studies have typically assumed Maxwell–Boltzmann statistics for neutron capture rates. To resolve this discrepancy, we present a modified Saha equation for nuclear statistical equilibrium that accounts for neutron degeneracy and incorporates the newly computed degenerate neutron capture rates (B. Knight et al.) into the WinNet nuclear reaction network. This correction significantly alters initial nuclear abundances, r -process nucleosynthesis pathways, and energy generation. Our results reveal that degenerate neutron capture induces cooling of the ejecta (reaching up to ∼10 55 erg s −1 ), followed by enhanced late-time heating driven by neutron captures (up to 10 54 erg s −1 ), beta decay, and neutron emissions. Strikingly, the influence of neutron degeneracy persists for nearly 1 s, suggesting potentially detectable observational signatures. These modifications can have implications for the light curves, possibly shaping their early-time luminosity and thermal evolution or even short gamma-ray bursts, which are believed to be associated with mergers. Nevertheless, our findings do not have a significant impact on the final abundances or the total energy.