New constraints on the Al25(p,γ) reaction and its influence on the flux of cosmic γ rays from classical nova explosions

The astrophysical $^{25}\mathrm{Al}(p,\ensuremath{\gamma})\phantom{\rule{0.16em}{0ex}}^{26}\mathrm{Si}$ 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 $^{25}\mathrm{Al}(p,\ensuremath{\gamma})$ 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 $19(3)\phantom{\rule{0.28em}{0ex}}\mathrm{fs}$ and provide compelling evidence for the existence of a ${1}^{\ensuremath{-}}$ state in the $T=1,\phantom{\rule{0.28em}{0ex}}A=26$ system, indicating a previously unaccounted for $\ensuremath{\ell}=1$ resonance in the $^{25}\mathrm{Al}(p,\ensuremath{\gamma})$ reaction. Using the presently measured lifetime, together with the assumption that the likely ${1}^{\ensuremath{-}}$ state corresponds to a resonance in the $^{25}\mathrm{Al}+p$ system at 435.7(53) keV, we find considerable differences in the $^{25}\mathrm{Al}(p,\ensuremath{\gamma})$ reaction rate compared to previous works. Based on current nova models, we estimate that classical novae may be responsible for up to $\ensuremath{\approx}15%$ of the observed galactic abundance of $^{26}\mathrm{Al}$.