Single-Nucleon Transfer on Unstable ^{59}Cu Probes the NiCu Cycle in Astrophysical X-Ray Bursts

Recent models of the rapid proton (rp) capture process indicate that a competition between the ^{59}Cu(p,γ)^{60}Zn and ^{59}Cu(p,α)^{56}Ni reactions may result in the formation of a nickel-copper (NiCu) cycle that traps the flux of material between ^{56}Ni and ^{60}Zn. Here, we report the identification of 15 proton-unbound levels in ^{60}Zn, populated via ^{59}Cu(d,n) transfer, which govern the rate of the ^{59}Cu(p,γ)^{60}Zn reaction in XRBs. Precise excitation energies for levels in ^{60}Zn were obtained from observed γ decays, and spectroscopic factors were determined from angle-integrated cross sections. Incorporating these results into stellar-model calculations, we find that with experimentally constrained uncertainties a NiCu cycle in XRBs is indeed possible, though we limit its branching strength to less than 38%. While modest, such a branching has significant impact on the light curve, motivating further studies of the relevant rates. Our calculations also indicate that a significant NiCu cycle leads to an increase in the amount of odd-A nuclei in the burst ashes, which may affect Urca cooling processes in neutron star crusts.