The upper envelope of the amplitude of the VLBI visibility function usually represents the most compact structural pattern of extragalactic radio sources , in particular , the “ core-jet ” morphologies .
By fitting the envelope to a circular Gaussian model in \sim 3000 parsec-scale core-jet structures , we find that the apparent angular size shows significant power-law dependence on the observing frequency ( power index n = -0.95 \pm 0.37 ) .
The dependence is likely to result from synchrotron self-absorption in the inhomogeneous jet and not the free-free absorption ( n = -2.5 ) , nor the simple scatter broadening ( n \leq - 2 ) .