New constraints on the ${}^{25}\mathrm{Al}(p,\gamma )$ reaction and its influence on the flux of cosmic $\gamma$ rays from classical nova explosions

The astrophysical ${}^{25}\mathrm{Al}(p,\gamma ){}^{26}\mathrm{Si}$ reaction represents one of the key remaining uncertainties in accurately modeling the abundance of radiogenic ${}^{26}\mathrm{Al}$ ejected from classical novae. Specifically, the strengths of key proton-unbound resonances in ${}^{26}\mathrm{Si}$, that govern the rate of the ${}^{25}\mathrm{Al}(p,\gamma )$ reaction under explosive astrophysical conditions, remain unsettled. Here, we present a detailed spectroscopy study of the ${}^{26}\mathrm{Si}$ mirror nucleus ${}^{26}\mathrm{Mg}$. We have measured the lifetime of the $3^{+}$, 6.125-MeV state in ${}^{26}\mathrm{Mg}$ to be $19\left(3\right)\mathrm{fs}$ and provide compelling evidence for the existence of a $1^{-}$ state in the $T=1,A=26$ system, indicating a previously unaccounted for $\ell =1$ resonance in the ${}^{25}\mathrm{Al}(p,\gamma )$ reaction. Using the presently measured lifetime, together with the assumption that the likely $1^{-}$ state corresponds to a resonance in the ${}^{25}\mathrm{Al}+p$ system at 435.7(53) keV, we find considerable differences in the ${}^{25}\mathrm{Al}(p,\gamma )$ reaction rate compared to previous works. Based on current nova models, we estimate that classical novae may be responsible for up to $\approx 15\%$ of the observed galactic abundance of ${}^{26}\mathrm{Al}$.

Figure 1

Centroid peak positions of the 1775- (red arrows) and 1809-keV (blue arrows) $\gamma$ rays with a gate placed on the 2541-keV $3_2^{+}\to 2_1^{+}$ transition at angles of ${37}^{\circ },{90}^{\circ }$, and ${143}^{\circ }$, respectively. (The inset) Observed angular distribution of the 1775-keV $\gamma$ ray, corresponding to the decay of the $3^{+}$ 6125-keV state in ${}^{26}\mathrm{Mg}$ to the $3^{+}$ 4350-keV level.

Figure 2

$\gamma \text{−}\gamma \text{−}\gamma$ coincidence spectrum with gates placed at 1809 and 1778 keV, respectively. Transitions associated with decays to the $0_2^{+}$ 3587-keV level in ${}^{26}\mathrm{Mg}$ are denoted by red asterisks. We note that further known decays in ${}^{26}\mathrm{Mg}$ [38], not associated with the $0_2^{+}$ state, are also observed. This is due to a number of excited states in ${}^{26}\mathrm{Mg}$ exhibiting $\gamma$-decay branches with energies $\approx 1778\mathrm{keV}$ [38]. (The inset) Expanded view of the energy region of the 2123-keV transition. Using fixed peak widths for observed $\gamma$-ray transitions, a double peak fit is necessary to account for the width of the 2123/ 2133-keV doublet.

Figure 3

(Top) Differential cross section as a function of the laboratory angle for the 5710/5716-keV doublet state in ${}^{26}\mathrm{Mg}$. (Bottom) Angular distribution of the 6125-keV state in ${}^{26}\mathrm{Mg}$. The expected $\ell =0[C^{2}S=0.14\left(3\right)]$ and $\ell =2[C^{2}S=0.30\left(6\right)]$ distributions for this $3^{+}$ level are clearly observed.