Nucleosynthesis constraints through gamma-ray line measurements from classical novae. A hierarchical model for the ejecta of 22Na and 7Be

Context. Classical novae belong to the most frequent transient events in the Milky Way and are key agents of ongoing nucleosynthesis. Despite their large numbers, they have never been observed in soft γ -ray emission. Measurements of their γ -ray signatures would provide insights into explosion mechanism and nucleosynthesis products.Aims. Our goal is to constrain the ejecta masses of 7 Be and 22 Na from classical novae through their γ -ray line emissions at 478 and 1275 keV.Methods. We extracted posterior distributions on the line fluxes from archival data of the INTEGRAL/SPI spectrometer telescope. We then used a Bayesian hierarchical model to link individual objects and diffuse emission, and to infer ejecta masses from the whole population of classical novae in the Galaxy.Results. Individual novae are too dim to be detectable in soft γ -rays, and the upper bounds on their flux and ejecta mass uncertainties cover several orders of magnitude. Within the framework of our hierarchical model, we can nevertheless infer tight upper bounds on the 22 Na ejecta masses, given all uncertainties from individual objects as well as diffuse emission, of M ⊙ (99.85th percentile).Conclusions. In the context of ONe nucleosynthesis, the 22 Na bounds are consistent with theoretical expectations and exclude that most ONe novae occur on white dwarfs with masses of about 1.35 M ⊙ . The upper bounds from 7 Be are uninformative. From the combined ejecta mass estimate of 22 Na and its β + decay, we infer a positron production rate of  e+  s−1 , which would mean 10% at most of the total annihilation rate in the Milky Way.