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Classical-Nova Contribution to the Milky Way's $^{26}$Al Abundance: Exit Channel of the Key $^{25}$Al($p,γ$)$^{26}$Si Resonance

Classical novae are expected to contribute to the 1809-keV Galactic $γ$-ray emission by producing its precursor $^{26}$Al, but the yield depends on the thermonuclear rate of the unmeasured $^{25}$Al($p,γ$)$^{26}$Si reaction. Using the $β$ decay of $^{26}$P to populate the key $J^π=3^+$ resonance in this reaction, we report the first evidence for the observation of its exit channel via a $1741.6 \pm 0.6 (\textrm{stat}) \pm 0.3 (\textrm{syst})$ keV primary $γ$ ray, where the uncertainties are statistical and systematic, respectively. By combining the measured $γ$-ray energy and intensity with other experimental data on $^{26}$Si, we find the center-of-mass energy and strength of the resonance to be $E_r = 414.9 \pm 0.6(\textrm{stat}) \pm 0.3 (\textrm{syst}) \pm 0.6(\textrm{lit.})$ keV and $ωγ= 23 \pm 6 (\textrm{stat})^{+11}_{-10}(\textrm{lit.})$ meV, respectively, where the last uncertainties are from adopted literature data. We use hydrodynamic nova simulations to model $^{26}$Al production showing that these measurements effectively eliminate the dominant experimental nuclear-physics uncertainty and we estimate that novae may contribute up to 30% of the Galactic $^{26}$Al.