Broadband lepto-hadronic SED of the M31 central source: data points are shown as dots (with errorbars indicating frequency and flux uncertainties), model predictions as curves (top). The galaxy-wide flux densities of Battistelli et al. (2019) have been renormalised to the region encompassed by R_⋆ (i.e., ∼15% of the total emission; see Sect. 4.1.2). Emission components are denoted by these line types: radio synchrotron, dotted; thermal free–free emission, dot-dashed; total radio, solid; Comptonised CMB, short-dashed; Comptonised starlight (EBL+FGL), long-dashed; non-thermal bremsstrahlung, dotted; pionic, solid. Leptonic emission includes that from primary and secondary electrons(secondary components are dominant). In the X-ray/γ-ray range, on top of the separate components, a thick solid line indicates the total emission. Same for a broadband PSR+LH SED (middle). The 500 pulsars emission component (see text) is ahown as a dot-dashed line. Same as above but for 1000 pulsars (bottom). Note: in this case the secondary leptonic components are subdominant.