Influence of ${}^{73}\mathrm{Rb}$ on the ashes of accreting neutron stars

We find that the proton separation energy, $S\left(p\right)$, of ${}^{73}\mathrm{Rb}$ is $-640\left(40\right)$ keV, deduced from the observation of $\beta$-delayed ground-state protons following the decay of ${}^{73}\mathrm{Sr}$. This lower-limit determination of the proton separation energy of ${}^{73}\mathrm{Rb}$ coupled with previous upper limits from nonobservation, provides a full constraint on the mass excess with $\mathrm{\Delta }M{(}^{73}\text{Rb})=-46.01\pm 0.04$ MeV. With this new mass excess and the excitation energy of the $J^{\pi }=5/2^{-}$ isobaric-analog state ($T=3/2$) in ${}^{73}\mathrm{Rb}$, an improved constraint can be put on the mass excess of ${}^{73}\mathrm{Sr}$ using the isobaric-multiplet mass equation (IMME), and we find $\mathrm{\Delta }M{(}^{73}\text{Sr})=-31.98\pm 0.37$ MeV. These new data were then used to study the composition of ashes on accreting neutron stars following Type I x-ray bursts. Counterintuitively, we find that there should be an enhanced fraction of $A>102$ nuclei with more negative proton separation energies at the ${}^{72}\mathrm{Kr}$ rp-process waiting point. Larger impurities of heavier nuclei in the ashes of accreting neutron stars will impact the cooling models for such astrophysical scenarios.

Figure 1

The solid black histogram shows the $\beta$-delayed proton spectrum of ${}^{73}\mathrm{Sr}$. The fits to the proton decay of ${}^{73}\mathrm{Rb}^{*}$(IAS) measured previously, as well as the peaks from the Bayesian analysis performed on the unbinned data are shown as a solid red line. The residuals taken from subtracting the measured background (indicated by the blue shaded region) and fits are also shown. The inset shows the particular region of interest (ROI) for this study.

Figure 2

A corner plot of the Bayesian analysis performed on the ROI showing the (a) two-peak (doublet) and (b) three-peak (triplet) model. The off-diagonals show the correlations between the parameters.

Figure 3

The Bayes factor for each model was calculated by thermodynamic annealing. The resulting $\langle E_L\rangle$ for the different temperature parameters used is presented.

Figure 4

The 1-$\sigma$ confidence interval (red shaded area) from the IMME fit using the measured proton separation energy of ${}^{73}\mathrm{Rb}$ as well as the empirically deduced distribution of mirror energy differences. The black triangles and error bars, show the limits used in the IMME fit, while the blue square shows the current limits for the mass excess of ${}^{73}\mathrm{Sr}$ from the AME.

Figure 5

(a) Ashes from XRB model calculations for $S\left(p\right)$ of ${}^{73}\mathrm{Rb}$ $\pm 3\sigma$ for the 2016 AME (black) and our result $\pm 3\sigma$ (red). (b) Ratio of the results from (a) to the $X\left(A\right)$ of calculations with $S\left(p\right)=30$ keV for high-$A$ ashes. Our result overlaps the lower $|S_p|$ portion of the AME predictions. Uncertainty bands include fluctuations due to burst-to-burst scatter near the end of the burst sequence.