Correlation between the luminosity at 220 GHz and the black hole mass. The points and the blue curve are taken from Ruffa et al. (2024); filled circles show sources with dynamical M_BH measurements, and open squares represent sources with M_BH derived using the M_BH–σ_⋆ relation (Ruffa et al. 2024). The upper limits are plotted in green for clarity. We also plot the sources we studied with pink stars (except for NGC 3227, which was already included in their sample). The colour-gradient curves are the corona luminosities calculated for the model parameters defined in Sect. 3.1, and using the approximate mean values of r_c = 130 and log δ = −2.0 (Sect. 3.3). We explore different values of r_c (top panel) and δ (bottom panel), covering the ranges derived for the sources in the sample.