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

Abstract

During the X-ray bursts of GS 1826−24, a “clocked burster”, the nuclear reaction flow that surges through the rapid-proton capture process path has to pass through the NiCu cycles before reaching the ZnGa cycles that moderate further hydrogen burning in the region above the germanium and selenium isotopes. The 57Cu(p,γ)58Zn reaction that occurs in the NiCu cycles plays an important role in influencing the burst light curves found by Cyburt et al. We deduce the 57Cu(p,γ)58Zn reaction rate based on the experimentally determined important nuclear structure information, isobaric-multiplet-mass equation, and large-scale shell-model calculations. Based on the isobaric-multiplet-mass equation, we propose a possible order of - and -dominant resonance states and constrain the resonance energy of the state. The latter reduces the contribution of the -dominant resonance state. The new reaction rate is up to a factor of 4 lower than the Forstner et al. rate recommended by JINA REACLIB v2.2 at the temperature regime sensitive to clocked bursts of GS 1826−24. Using the simulation from the one-dimensional implicit hydrodynamic code Kepler to model the thermonuclear X-ray bursts of the GS 1826−24 clocked burster, we find that the new 57Cu(p,γ)58Zn reaction rate, coupled with the latest 56Ni(p,γ)57Cu and 55Ni(p,γ)56Cu reaction rates, redistributes the reaction flow in the NiCu cycles and strongly influences the burst ash composition, whereas the 59Cu(p,α)56Ni and 59Cu(p,γ)60Zn reactions suppress the influence of the 57Cu(p,γ)58Zn reaction and diminish the impact of nuclear reaction flow that bypasses the important 56Ni waiting point induced by the 55Ni(p,γ)56Cu reaction on the burst light curve.