Direct Measurement of 59Cu(p,α)56Ni Precludes a Strong NiCu Cycle in Type-I X-Ray Bursts

Model-observation comparisons of type-I X-ray bursts (XRBs) can reveal the properties of accreting neutron star systems, including the neutron star compactness. XRBs are powered by nuclear burning, and a handful of reactions have been shown to impact the model results. Reactions in the NiCu cycles, featuring a competition between 59Cu(p,γ)60Zn and 59Cu(p,α)56Ni, have been shown to be among the most important reactions as they are a critical checkpoint in rp-process flow and significantly impact the light curves and burst ashes. We report a direct measurement of 59Cu(p,α)56Ni, bringing stringent constraints on this reaction rate. New results rule out a strong NiCu cycle in XRBs, with a negligible degree of recycling, ≤5% up to 1.5 GK. The new reaction rate, when varied within new uncertainty limits, shows no impact on one-zone XRB model light curves tailored for clocked-burster GS1826–24, hence removing an important nuclear physics uncertainty in the model-observation comparison.