New constraints on the \nAl25(p,γ)\n reaction and its influence on the flux of cosmic \nγ\n rays from classical nova explosions

Abstract

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 $^{26}\\mathrm{Al}$.