The Regulated GeAs Cycles with the New ⁶³ Ga(p, <i>γ</i> ) ⁶⁴ Ge and ⁶⁴ Ge(p, <i>γ</i> ) ⁶⁵ As Reaction Rates and Their Impact on the GS 1826−24 Clocked Bursts and SAX J1808.4−3658 Photospheric Radius Expan

Abstract The 63 Ga(p, γ ) 64 Ge and 64 Ge(p, γ ) 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, γ ) 64 Ge and 64 Ge(p, γ ) 65 As reaction rates to the state-of-the-art self-consistent one-dimensional multizone thermohydrodynamic code K epler 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. 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. With or without the inclusion of the newly determined reaction rate of the highly influential 22 Mg( α , p) 25 Al reaction, the impact of the new 63 Ga(p, γ ) 64 Ge and 64 Ge(p, γ ) 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 the SAX J1808.4−3658 photospheric radius expansion burster are more sensitive to the uncertainties of thermonuclear reaction rates.