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

TL;DR

This work tackles whether a strong NiCu cycle operates during Type-I X-ray bursts by directly measuring the rate of the crucial proton-induced exit $^{59}$Cu$(p,\alpha)^{56}$Ni. Using inverse kinematics with a $^{59}$Cu beam at TRIUMF/ISAC on a solid $\mathrm{H}_2$ target, the authors extract cross sections at $E_{cm}=4.0\pm0.4$ and $4.68\pm0.25$ MeV and reconstruct the $^{56}$Ni excitation spectrum, finding a ground-state-dominated cross section with no first-excited-state signal. The resulting rate is about a factor of $0.5$ the NON-SMOKER prediction (uncertainty ~2), and reveals NiCu recycling is negligible ($\leq 5\%$ up to $T=1.5$ GK), precluding a strong NiCu cycle. In a one-zone XRB model, the rate variation within the new uncertainty does not affect the light curve, thereby removing a major nuclear physics uncertainty and enabling more robust model$-$observation comparisons to constrain neutron star properties.

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($p$,$γ$)$^{60}$Zn and $^{59}$Cu($p$,$α$)$^{56}$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($p$,$α$)$^{56}$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.

Figures (5)

• Figure 1: Particle Identification plot, using $\Delta$E-E detectors, at E$_{c.m.}$ 4.68$\pm$0.25 MeV
• Figure 2: The excitation energy spectrum of $^{56}$Ni with (blue) and without (red) H$_{\rm{{2}}}$ target at E$_{c.m.}$ 4.0$\pm$0.4 MeV (upper) and 4.68$\pm$0.25 MeV (lower panel), respectively.
• Figure 3: Experimentally determined angular distributions at E$_{c.m.}$ 4.0 MeV (upper panel), and at 4.68 MeV (lower panel). TALYS calculations (for different $\alpha$-OMPs) and Legendre polynomial fits to data are shown at both energies.
• Figure 4: Top Panel: Experimental cross sections at the weighted center-of-mass energies are shown in comparison to default NON-SMOKER cross sections (blue curve). Lower Panel: New recommended reaction rate and associated uncertainty band is shown in comparison to REACLIB rate which is based on Non-Smoker calculations. Lower panel inset shows REACLIB rate coefficients for recommended rate.
• Figure 5: Top Panel: Ratio of $^{59}$Cu(p,$\alpha$)$^{56}$Ni (recommended rate, this work) to $^{59}$Cu(p,$\gamma$)$^{60}$Zn from two different works, depicting NiCu cycle strength in XRBs. Temperatures of interest for XRBs is highlighted by the yellow band. Lower panel: Impact of reaction rate variation on one-zone X-ray burst light curves, with ignition conditions curated for X-ray binary $\tt{GS 1826-24}$. New rate, considering upper-limit, is factor 2 uncertain, and this change in reaction rate does not impact the light curve shape.