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

Abstract

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,γ)56Ni and 59Cu(p,α)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.