  Simulations of explosive nucleosynthesis in novae predict the production of
$^{22}$Na, a key astronomical observable to constrain nova models. Its
gamma-ray line at 1.275 MeV has not yet been observed by the gamma-ray space
telescopes. The $^{20}$Ne/$^{22}$Ne ratio in presolar grains, a possible tool
to identify nova grains, also depends on $^{22}$Na produced. Uncertainties on
its yield in classical novae currently originate from the rate of the
$^{22}$Na(p, $\gamma$)$^{23}$Mg reaction. At peak novae temperatures, this
reaction is dominated by a resonance at E$_{\text{R}}$=0.204 MeV, corresponding
to the $E_x$=7.785 MeV excited state in $^{23}$Mg. The resonance strengths
measured so far disagree by one order of magnitude. An experiment has been
performed at GANIL to measure the lifetime and the proton branching ratio of
this key state, with a femtosecond resolution for the former. The reactions
populating states in $^{23}$Mg have been studied with a high resolution
detection set-up, i.e. the particle VAMOS, SPIDER and gamma tracking AGATA
spectrometers, allowing the measurements of lifetimes and proton branchings. We
present here a comparison between experimental results and shell-model
calculations, that allowed us to assign the spin and parity of the key state.
Rather small values obtained for reduced $M1$ matrix elements, $|M(M1)|\lesssim
0.5$ $\mu_N$, and proton spectroscopic factors, $C^{2}S_{\text{p}}$<10$^{-2}$,
seem to be beyond the accuracy of the shell model. With the reevaluated
$^{22}$Na(p, $\gamma$)$^{23}$Mg rate, the $^{22}$Na detectability limit and its
observation frequency from novae are found promising for the future space
telescopes.
