A special case of optically thick radio source are flat spectrum radio sources. Current understanding is that such sources result as a superposition of a number of self absorbed sources of different sizes (onion skin model) which self absorb at different frequencies. The figure 5.6 shows the flat spectrum source formed by many different self absorbed components. Even these sources sometimes shows turn over at low frequencies which can be used to estimate the properties of the outermost layers. The figure 5.7 shows the flat spectrum source with low frequency turnover.
