Predicted variability of extragalactic radio sources experiencing long-term injection of relativistic electrons

On the basis of Kuril'chik's model B (magnetic energy density predominant) for the variable emission of an extragalactic radio source, theoretical relations are obtained to describe the changes in spectral flux density when relativistic electrons are injected over a protracted period. A preliminary comparison of the theory with centimeter-wavelength measurements of the variable quasar 3 C 345 indicates satisfactory agreement.