First Direct Measurement of ${}^{22}\mathrm{Mg}(\alpha ,p){}^{25}\mathrm{Al}$ and Implications for X-Ray Burst Model-Observation Comparisons

Type-I x-ray bursts can reveal the properties of an accreting neutron star system when compared with astrophysics model calculations. However, model results are sensitive to a handful of uncertain nuclear reaction rates, such as ${}^{22}\mathrm{Mg}(\alpha ,p)$. We report the first direct measurement of ${}^{22}\mathrm{Mg}(\alpha ,p)$, performed with the Active Target Time Projection Chamber. The corresponding astrophysical reaction rate is orders of magnitude larger than determined from a previous indirect measurement in a broad temperature range. Our new measurement suggests a less-compact neutron star in the source GS1826-24.

Figure 1

Panel (a) shows a three-dimensional view of an example proton track and the arrow shows the beam direction. Panel (b) shows the projection of a proton track on the pad plane where blue dots are data points and red indicates the data points chosen for RANSAC analysis. Panel (c) shows the arclength of each hit pattern point as a function of the $z$ coordinate. The red line is the least-squares fit performed to extract the scattering angle from the slope.

Figure 2

Panel (a) shows the reaction vertex as a function of laboratory angle for experimental data (blue inverted triangle) overlaid on simulation data (red filled circles). Panel (b) shows the efficiency as a function of laboratory angle and the inset shows the number of pads hit per event as a function of the laboratory angle. Panel (c) shows the ratio of calculated (using PACE4) total counts over the counts in the experimental angular domain as a function of the beam energy.

Figure 3

Panel (a) shows the experimental cross sections obtained in the present work over a range of center-of-mass energies covered (black). For all the points, the cross section weighted energy is shown, which is the reason why horizontal error bars for the two lowest energy points are asymmetric. Panel (b) shows the reaction rate comparison of the current work to different model predictions and to the previous measurement by Matic et al. [11].

Figure 4

XRB light curve comparison for GS1826-24 observations (gray boxes), mesa calculations using the non-smoker rate for ${}^{22}\mathrm{Mg}(\alpha ,p)$ (black band), and the non-smoker rate reduced by $/8$ (red band). The inset shows the corresponding 68% (red) and 95% (yellow) confidence intervals for the distance and redshift, which were determined as described in Ref. [8].