Fig 1. A sequence of images of different frequencies (or times)
of the synchrotron emission from the same magnetic field configuration,
for a weak field model B = 0.3
or a
diffusive model with D = 0.943. The images at top left, top
right, bottom left, and bottom right are at frequencies 
/
= 0.03, 0.3, 3, and 30
respectively. The contour levels cover the full range of the pixel
values in the image, with the solid contours covering the top half of
the pixel values, and the dotted lines covering the fainter parts of
the image.
In a weak field, the dominant energy loss mechanism is inverse Compton scattering of microwave background photons. With strong diffusion, the energy losses are almost independent of the local field strength. The electron energy spectrum is then independent of position or field strength. The synchrotron emissivity depends on the local field strength, so that the low frequency image is simply mapping the magnetic field. At frequencies above the spectral break, the emissivity varies strongly as a function of local field strength because the field strength determines the position of the break. The bright regions stand out above the background because the magnetic field moves the break frequency to higher frequencies.
Fig 2. Histograms of pixel intensity (normalized to the mean intensity of each image) for the sequence of images shown in Fig. 1. Note that the shape changes from a roughly bell-shaped distribution at low frequencies to a low background with a tail of high intensity points at high frequency.
The distribution of intensity in the image changes with frequency. This is shown in Fig. 2, where it can be seen that at high frequency a tail of high intensity points develops, corresponding to those points where the field strength is high and the emission has not yet faded.
___________________________________ Peter Tribble, peter.tribble@gmail.com