Direct measurement of $^{59}$Cu($p$,$\alpha$)$^{56}$Ni precludes a strong NiCu cycle in Type-I X-ray bursts

TL;DR

This study provides a direct measurement of the $^{59}$Cu($p$,$eta$)$^{56}$Ni reaction rate, demonstrating that the NiCu cycle is negligible in type-I X-ray bursts and reducing nuclear physics uncertainties in burst models.

Contribution

The paper presents the first direct measurement of the $^{59}$Cu($p$,$eta$)$^{56}$Ni reaction rate, constraining the NiCu cycle's role in X-ray bursts.

Findings

The NiCu cycle is negligible in XRBs, with recycling $ extless$5%.

The new reaction rate does not affect XRB light-curve models.

The measurement constrains key nuclear reaction pathways in XRBs.

Abstract

Model-observation comparisons of type-I X-ray bursts (XRBs) can reveal the properties of accreting neutron star systems, including the neutron star compactness. XRBs are powered by nuclear burning and a handful of reactions have been shown to impact the model results. Reactions in the NiCu cycles, featuring a competition between $^{59}$Cu($p$,$\gamma$)$^{60}$Zn and $^{59}$Cu($p$,$\alpha$)$^{56}$Ni, have been shown to be among the most important reactions as they are a critical checkpoint in $rp$-process flow and significantly impact the light curves and burst ashes. We report a direct measurement of $^{59}$Cu($p$,$\alpha$)$^{56}$Ni bringing stringent constraints on this reaction rate. New results rule out a strong NiCu cycle in XRBs, with a negligible degree of recycling, $\leq$5\% up to 1.5 GK. The new reaction rate, when varied within new uncertainty limits, shows no impact on one-zone XRB model light-curves tailored for clocked-burster $\tt{GS 1826-24}$, hence removing an important nuclear physics uncertainty in the model-observation comparison.