Effects of Competing Reactions on $^{59}\mathrm{Cu}$ and $^{63}\mathrm{Ga}$ on Abundance Distributions in Type I X-Ray Bursts

Abstract

The composition of X-ray burst ashes affects the thermal transport properties of the neutron star crust, as well as heating and cooling through weak interaction processes. Reaction-rate uncertainties can produce secondary peaks separated from the main peak in final abundance distributions. Using STARLIB reaction rates and their temperature-dependent uncertainties in the K04 one-zone post-processing model, we combine four sets of Monte Carlo calculations with XGBoost classification, SHAP analysis, and reaction-flow analysis to investigate the origins of these secondary peaks. We find that the competing $(p,α)$ and $(p,γ)$ reaction pairs on $^{59}\mathrm{Cu}$ and $^{63}\mathrm{Ga}$ are primarily responsible for secondary peaks outside the central 68\% confidence interval in all 44 isotopes examined. Fixing both pairs at their median rates reduces the fractions of samples in the secondary-peak regions by 96\%--100\%, while the bounds of the central 68\% interval remain largely unchanged for most mass numbers. The $^{59}\mathrm{Cu}$ pair has the larger effect. Enhanced recycling limits the supply of material to heavier isotopes and accelerates proton consumption, while earlier proton exhaustion further suppresses subsequent proton captures. These changes alter the balance between isotope production and destruction, leading to high- or low-abundance secondary peaks over a broad mass range beyond the Ni--Cu and Zn--Ga cycles. These results identify $^{59}\mathrm{Cu}(p,γ)^{60}\mathrm{Zn}$ and the two competing channels on $^{63}\mathrm{Ga}$ as priorities for further nuclear-physics constraints to improve XRB abundance predictions.