Fig. 2: RML images and model images at 3.5 mm and 1.3 mm.

a–f, RML images (a,d) and model images (b,c,e,f) obtained at 3.5 mm (a–c) and 1.3 mm (d–f). a, The 3.5-mm image obtained on 14–15 April 2018 is the same as in Fig. 1c but shown on a linear brightness scale. b,e, The thermal synchrotron model from the accretion flow assumes synchrotron emission from electrons with a Maxwellian energy distribution. c,f, The non-thermal synchrotron model from the jet region assumes synchrotron emission from electrons with a power-law energy distribution. d, The 1.3-mm EHT image obtained on 11 April 2017, reconstructed with the publicly available data⁹ and imaging pipeline⁶ using the EHT-imaging library²⁶. Note that the differences in the azimuthal intensity distribution in the two observed images are probably because of time variability and/or blending effects with the underlying jet footpoints. Although the morphology of both models is consistent with the observations at 1.3 mm (e and f), the larger and thicker ring-like structure at 3.5 mm can be understood by the opacity effect at longer wavelengths²⁷, preferentially explained by thermal synchrotron absorption from the accretion flow region (b). For comparison, reconstructed and simulated images are convolved with a circular Gaussian beam of 27 μas (3.5 mm) and 10 μas (1.3 mm) and are shown in a linear colour scale. The blue circle denotes the measured ring diameter of 64 μas at 3.5 mm and 42 μas at 1.3 mm.