Impact of the New ⁶⁵As(p,γ)⁶⁶Se Reaction Rate on the Two-proton Sequential Capture of ⁶⁴Ge, Weak GeAs Cycles, and Type I X-Ray Bursts Such as the Clocked Burster GS 1826−24

Abstract

We reassess the 65As(p,γ)66Se reaction rates based on a set of proton thresholds of 66Se, S p(66Se), estimated from the experimental mirror nuclear masses, theoretical mirror displacement energies, and full p f-model space shell-model calculation. The self-consistent relativistic Hartree–Bogoliubov theory is employed to obtain the mirror displacement energies with much reduced uncertainty, and thus reducing the proton-threshold uncertainty up to 161 keV compared to the AME2020 evaluation. Using the simulation instantiated by the one-dimensional multi-zone hydrodynamic code, Kepler, which closely reproduces the observed GS 1826−24 clocked bursts, the present forward and reverse 65As(p,γ)66Se reaction rates based on a selected S p(66Se) = 2.469 ± 0.054 MeV, and the latest 22Mg(α,p)25Al, 56Ni(p,γ)57Cu, 57Cu(p,γ)58Zn, 55Ni(p,γ)56Cu, and 64Ge(p,γ)65As reaction rates, we find that though the GeAs cycles are weakly established in the rapid-proton capture process path, the 65As(p,γ)66Se reaction still strongly characterizes the burst tail end due to the two-proton sequential capture on 64Ge, not found by the Cyburt et al. sensitivity study. The 65As(p,γ)66Se reaction influences the abundances of nuclei A = 64, 68, 72, 76, and 80 up to a factor of 1.4. The new S p(66Se) and the inclusion of the updated 22Mg(α,p)25Al reaction rate increases the production of 12C up to a factor of 4.5, which is not observable and could be the main fuel for a superburst. The enhancement of the 12C mass fraction alleviates the discrepancy in explaining the origin of the superburst. The waiting point status of and two-proton sequential capture on 64Ge, the weak-cycle feature of GeAs at a region heavier than 64Ge, and the impact of other possible S p(66Se) are also discussed.

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

“…The period of this transient existence may depend on the precise determination of the S p ( 66 Se) value. The Present 57 Cu(p,γ) 58 Zn reaction rate, which is more constrained than Langer et al (2014) reaction rate, was used by Lam et al (2022) to study the weak GeAs cycles and was also recently used by Hu et al (2021) to study the prevailing influence of the newly deduced 22 Mg(α,p) 25 Al.…”

“…Meanwhile, the induced 57 Zn(β + ν) 57 surging through nuclei heavier than 68 Se. We find that the GeAs cycle that involves the two-proton sequential capture of 64 Ge consisting of 64 Ge(p,γ) 65 As(p,γ) 66 Se reactions could weakly exist in the middle of onset until the moment after burst peak (Lam et al 2022); see the nucleosynthesis charts in Figures 6, 7 We notice that the balance between the 56 Ni(p,γ) 57 Cu and 57 Cu(p,γ) 58 Zn reactions also redistributes the reaction flow to the NiCu II cycle and then the reaction flow eventually joins with the NiCu I cycle and branches out to the ZnGa cycles at the 60 Zn waiting point or follows the 60 Cu(p,γ) 61 Zn(p,γ) 62 Ga reactions branches out to the ZnGa II cycle. Then, the joint reaction flow surges through the proton-rich region heavier than 64 Ge where (p,γ) reactions actively burn hydrogen and intensify the rise of burst light curve from t = −10 s up to t = 0 s (burst peak).…”

Horizons: Nuclear Astrophysics in the 2020s and Beyond

“…This has led to long bursts being recognized as rp-process signatures, while short bursts are either helium flashes or powered by a limited rp-process [382,383]. With these advances recent one-dimensional models were also able to reproduce observed burst sequences and their response to changes in accretion rate [382,384,385,386]. However, such successes are limited to the so called "textbook burster," a single system with unusually regular bursts [387].…”

Impacts of the Direct Urca and Superfluidity inside a Neutron Star on Type I X-Ray Bursts and X-Ray Superbursts

“…Modeling of observed burst light curves, therefore, is subject to probing of the underlying properties of NS equations of state (EOSs), the accretion rate, and the composition of accreting matter. In addition, the uncertainty of nuclear reaction rates especially for αp and rp processes affects the modeling of burst light curves (Hu et al 2021;Lam et al 2022aLam et al , 2022bMeisel et al 2022). The dependence of the above physical parameters on X-ray burst light curves has been investigated by many theoretical works (Woosley et al 2004;Heger et al 2007;Cyburt et al 2016;Meisel 2018;Meisel et al 2019;Johnston et al 2020;Lam et al 2022b).…”