The Regulated GeAs Cycles with the New ${}^{63}$Ga(p,${\hat{I}}^3$)${}^{64}$Ge and ${}^{64}$Ge(p,${\hat{I}}^3$)${}^{65}$As Reaction Rates and Their Impact on the GS 1826$-$24 Clocked Bursts and SAX J1808.4$-$3658 Photospheric Radius Expansion Bursts

Abstract

The ${}^{63}$Ga(p,${\hat{I}}^3$)${}^{64}$Ge and ${}^{64}$Ge(p,${\hat{I}}^3$)${}^{65}$As thermonuclear reactions connect the ZnGa and GeAs cycles by diverting the flow of the rapid proton capture process from ${}^{63}$Ga to ${}^{65}$As. Changes in these two reaction rates regulate the ZnGa and GeAs cycles and may affect the modeled properties matching with the observed counterparts of a type I X-ray burster. We implement the latest ${}^{63}$Ga(p,${\hat{I}}^3$)${}^{64}$Ge and ${}^{64}$Ge(p,${\hat{I}}^3$)${}^{65}$As reaction rates to the state-of-the-art self-consistent one-dimensional multi-zone thermo-hydrodynamic code, KEPLER, to study the influence of these new reaction rates on the models of the GS 1826$-$24 clocked burster and SAX J1808.4$-$3658 photospheric radius expansion burster. Both new reaction rates obtained by Lu et al. [Phys. Rev. C 110, 065804 (2024)] are determined from complementing the experimental input with the nuclear spectroscopic information deduced from the full pf-shell space configuration-interaction shell-model calculations. By constraining the models on reproducing the observed burst peak, light-curve profile, fluence, and recurrence time, we find that the impact of the newly measured proton thresholds and respective proton-capture reactions on the burst light-curve profile of the GS 1826$-$24 clocked burster is, in fact, not as significant as claimed by Zhou et al. [Nat. Phys. 19, 1091 (2023)]. With or without the inclusion of the newly determined reaction rate of the highly influential ${}^{22}$Mg($\hat{I}\pm$,p)${}^{25}$Al reaction, the impact of the new ${}^{63}$Ga(p,${\hat{I}}^3$)${}^{64}$Ge and ${}^{64}$Ge(p,${\hat{I}}^3$)${}^{65}$As reaction rates on SAX J1808.4$-$3658 photospheric radius expansion bursts is evident. Our finding indicates that the models reproducing the 2002 October epoch of SAX J1808.4$-$3658 photospheric radius expansion burster is more sensitive to the uncertainties of thermonuclear reaction rates.

12 pages, 6 figures (colorblind-friendly colors), 1 table, finalized version; first version was submitted on 29 Nov 2023, the only revised version was submitted on 28 Apr 2025; accepted by The Astrophysical Journal on 25 May 2025. To appear at The Astrophysical Journal (Open Access) on 28 Oct 2025