Determining the rp-process flow through 56Ni: resonances in 57Cu(p,γ)58Zn identified with GRETINA

Regulated NiCu Cycles with the New 57Cu(p,γ)58Zn Reaction Rate and the Influence on Type-I X-Ray Bursts: GS 1826–24 Clocked Burster

In Type-I X-ray bursts (XRBs), the rapid-proton capture (rp-) process passes through the NiCu and ZnGa cycles before reaching the region above Ge and Se isotopes that hydrogen burning actively powers the XRBs. The sensitivity study performed by Cyburt et al. [1] shows that the 57Cu(p,γ)58Zn reaction in the NiCu cycles is the fifth most important rp-reaction influencing the burst light curves. Langer et al. [2] precisely measured some low-lying energy levels of 58Zn to deduce the 57Cu(p,γ)58Zn reaction rate. Nevertheless, the order of the 1+1 and 2+3 resonance states that dominate at 0:2 ≲ T(GK) ≲ 0:8 is not confirmed. The 1+2 resonance state, which dominates at the XRB sensitive temperature regime 0:8 ≲ T(GK) ≲ 2 was not detected. Using isobaric-multipletmass equation (IMME), we estimate the order of the 1+1 and 2+3 resonance states and estimate the lower limit of the 1+2 resonance energy. We then determine the 57Cu(p,γ)58Zn reaction rate using the full pf -model space shell model calculations. The new rate is up to a factor of four lower than the Forstner et al. [3] rate recommended by JINA REACLIBv2.2. Using the present 57Cu(p,γ)58Zn, the latest 56Ni(p,γ)57Cu and 55Ni(p,γ)56Cu reaction rates, and 1D implicit hydrodynamic Kepler code, we model the thermonuclear XRBs of the clocked burster GS 1826–24. We find that the new rates regulate the reaction flow in the NiCu cycles and strongly influence the burst-ash composition. The 59Cu(p,γ)56Ni and 59Cu(p,α)60Zn reactions suppress the influence of the 57Cu(p,γ)58Zn reaction. They strongly diminish the impact of the nuclear reaction flow that bypasses the 56Ni waiting point induced by the 55Ni(p,γ)56Cu reaction on burst light curve.

Constraining the Neutron Star Compactness: Extraction of the ^{23}Al(p,γ) Reaction Rate for the rp Process

The ^{23}Al(p,γ)^{24}Si reaction is among the most important reactions driving the energy generation in type-I x-ray bursts. However, the present reaction-rate uncertainty limits constraints on neutron star properties that can be achieved with burst model-observation comparisons. Here, we present a novel technique for constraining this important reaction by combining the GRETINA array with the neutron detector LENDA coupled to the S800 spectrograph at the National Superconducting Cyclotron Laboratory. The ^{23}Al(d,n) reaction was used to populate the astrophysically important states in ^{24}Si. This enables a measurement in complete kinematics for extracting all relevant inputs necessary to calculate the reaction rate. For the first time, a predicted close-lying doublet of a 2_{2}^{+} and (4_{1}^{+},0_{2}^{+}) state in ^{24}Si was disentangled, finally resolving conflicting results from two previous measurements. Moreover, it was possible to extract spectroscopic factors using GRETINA and LENDA simultaneously. This new technique may be used to constrain other important reaction rates for various astrophysical scenarios.

High-Precision Mass Measurement of ^{56}Cu and the Redirection of the rp-Process Flow

We report the mass measurement of ^{56}Cu, using the LEBIT 9.4 T Penning trap mass spectrometer at the National Superconducting Cyclotron Laboratory at Michigan State University. The mass of ^{56}Cu is critical for constraining the reaction rates of the ^{55}Ni(p,γ) ^{56}Cu(p,γ) ^{57}Zn(β^{+}) ^{57}Cu bypass around the ^{56}Ni waiting point. Previous recommended mass excess values have disagreed by several hundred keV. Our new value, ME=-38626.7(7.1)  keV, is a factor of 30 more precise than the extrapolated value suggested in the 2012 atomic mass evaluation [Chin. Phys. C 36, 1603 (2012)CPCHCQ1674-113710.1088/1674-1137/36/12/003], and more than a factor of 12 more precise than values calculated using local mass extrapolations, while agreeing with the newest 2016 atomic mass evaluation value [Chin. Phys. C 41, 030003 (2017)CPCHCQ1674-113710.1088/1674-1137/41/3/030003]. The new experimental average, using our new mass and the value from AME2016, is used to calculate the astrophysical ^{55}Ni(p,γ) and ^{56}Cu(p,γ) forward and reverse rates and perform reaction network calculations of the rp process. These show that the rp-process flow redirects around the ^{56}Ni waiting point through the ^{55}Ni(p,γ) route, allowing it to proceed to higher masses more quickly and resulting in a reduction in ashes around this waiting point and an enhancement to higher-mass ashes.

Mass Measurement of 56Sc Reveals a Small A = 56 Odd-Even Mass Staggering, Implying a Cooler Accreted Neutron Star Crust

We present the mass excesses of (52-57)Sc, obtained from recent time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory at Michigan State University. The masses of 56Sc and 57Sc were determined for the first time with atomic mass excesses of -24.85(59)((-54)(+0))  MeV and -21.0(1.3)  MeV, respectively, where the asymmetric uncertainty for 56Sc was included due to possible contamination from a long-lived isomer. The 56Sc mass indicates a small odd-even mass staggering in the A = 56 mass chain towards the neutron drip line, significantly deviating from trends predicted by the global FRDM mass model and favoring trends predicted by the UNEDF0 and UNEDF1 density functional calculations. Together with new shell-model calculations of the electron-capture strength function of 56Sc, our results strongly reduce uncertainties in model calculations of the heating and cooling at the 56Ti electron-capture layer in the outer crust of accreting neutron stars. We find that, in contrast to previous studies, neither strong neutrino cooling nor strong heating occurs in this layer. We conclude that Urca cooling in the outer crusts of accreting neutron stars that exhibit superbursts or high temperature steady-state burning, which are predicted to be rich in A≈56 nuclei, is considerably weaker than predicted. Urca cooling must instead be dominated by electron capture on the small amounts of adjacent odd-A nuclei contained in the superburst and high temperature steady-state burning ashes. This may explain the absence of strong crust Urca cooling inferred from the observed cooling light curve of the transiently accreting x-ray source MAXI J0556-332.