Posterior distributions for diffuse emission of the 1275 keV line. The flux (top right) is separated into its two determining components, the ²²Na ejecta mass (top left) and the ONe nova rate (bottom right, treated as a nuisance parameter). Theoretical predictions are indicated in green from Starrfield et al. (2009). The joint posterior (68.3th, 95.4th, and 99.7th percentile) of ejecta mass and nova rate (bottom left) shows the expected anticorrelated behaviour according to Eq. (17).