The observed images, at various frequencies, are shown in Fig. 1 (GPSa, GPSb). For a thin bright ring appears on the outer edge of the image, while the center is much dimmer (only a few percent of the maximal surface brightness). For the surface brightness at the center is of the maximal surface brightness, and of the average surface brightness. For the surface brightness at the center of the image is of its average value, resulting in an almost uniform disk.
Copyright The European Southern Observatory (ESO)