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Blandford & Königl model the supersonic relativistic jet as a conical flow of electrons with energies distributed according to a power law distribution as would arise by acceleration in shocks. At increasing radii the density of electrons will decrease due to adiabatic expansion, and also the magnetic field strength decreases. Importantly though the model assumes that the electrons do not lose energy through radiation or expansion, due to some unexplained re-energisation process.
For a given frequency range νmin to νmax, there will be a frequency-dependent position along the jet whereby the emission becomes optically thin due to the electron density decreasing. Hence if the emission region covers the complete range of these positions for all ν then we will see the full range optical depths at all ν.
The lower region of the jet is optically thick to radiation of all frequencies. In the middle region, the jet becomes optically thin to radiation of progressively lower frequencies. The radiation frequency to which the jet becomes optically thin decreases until upper horizontal line whereby all the frequencies are optically thin.