2 The observed image

  

Figure 1: The observed image of a GRB afterglow at a given observed time, for different frequencies

The observed images, at various frequencies, are shown in Fig. 1 (GPSa, GPSb). For $\nu \gg \nu_{\rm m}$ 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 $\nu_{\rm a} \ll \nu \ll \nu_{\rm m}$ the surface brightness at the center is $34\%$ of the maximal surface brightness, and $58\%$ of the average surface brightness. For $\nu \ll \nu_{\rm a}$ the surface brightness at the center of the image is $77\%$ of its average value, resulting in an almost uniform disk.