Critical γ-ray compactness as a function of ϵ_γ when (i) σ_γγ and synchrotron emissivity are approximated by a Θ- and a δ-function respectively; and (ii) when for both quantities the full expressions are used. For the first case the numerical result (points) and the analytical one given by Eq. (34) (solid line) are shown. The full problem can only be treated numerically and the corresponding result is shown with a dashed line. The size of the source is R = 3 × 10¹⁶ cm and the magnetic field strength B = 3.57 G.