Regulated NiCu Cycles with the New ⁵⁷Cu(p,γ)⁵⁸Zn Reaction Rate and the Influence on Type-I X-Ray Bursts: GS 1826−24 Clocked Burster

In Type-I X-ray bursts (XRBs), the rapid-proton capture (rp-) process passes through the NiCu and ZnGa cycles before reaching the region above Ge and Se isotopes that hydrogen burning actively powers the XRBs. The sensitivity study performed by Cyburt et al. [1] shows that the ⁵⁷Cu(p,γ)⁵⁸Zn reaction in the NiCu cycles is the fifth most important rp-reaction influencing the burst light curves. Langer et al. [2] precisely measured some low-lying energy levels of ⁵⁸Zn to deduce the ⁵⁷Cu(p,γ)⁵⁸Zn reaction rate. Nevertheless, the order of the 1⁺₁ and 2⁺₃ resonance states that dominate at 0.2 ≲ T(GK) ≲ 0.8 is not confirmed. The 1⁺₂ resonance state, which dominates at the XRB sensitive temperature regime 0.8 ≲ T(GK) ≲ 2 was not detected. Using isobaric-multipletmass equation (IMME), we estimate the order of the 1⁺₁ and 2⁺₃ resonance states and estimate the lower limit of the 1⁺₂ resonance energy. We then determine the ⁵⁷Cu(p,γ)⁵⁸Zn reaction rate using the full pf-model space shell model calculations. The new rate is up to a factor of four lower than the Forstner et al. [3] rate recommended by JINA REACLIBv2.2. Using the present ⁵⁷Cu(p,γ)⁵⁸Zn, the latest ⁵⁶Ni(p,γ)⁵⁷Cu and ⁵⁵Ni(p,γ)⁵⁶Cu reaction rates, and 1D implicit hydrodynamic Kepler code, we model the thermonuclear XRBs of the clocked burster GS 1826−24. We find that the new rates regulate the reaction flow in the NiCu cycles and strongly influence the burst-ash composition. The ⁵⁹Cu(p,α)⁵⁶Ni and ⁵⁹Cu(p,γ)⁶⁰Zn reactions suppress the influence of the ⁵⁷Cu(p,γ)⁵⁸Zn reaction. They strongly diminish the impact of the nuclear reaction flow that bypasses the ⁵⁶Ni waiting point induced by the ⁵⁵Ni(p,γ)⁵⁶Cu reaction on burst light curve.