Time variation observations by Fender & Pooley of the microquasar GRS 1915+105 show quasi-periodic oscillations during the flare state of the jet. Furthermore these oscillations seem to show that a flat spectrum from 1011 Hz to 1014 Hz persist throughout the oscillation, with no detectable time lag in 1000 s flares.
The BK79 model was derived for a steady jet; this is not steady behaviour and yet the model is still being used to explain the oscillating flat spectrum. To achieve a flux increase uniformly across the entire flat spectrum requires that the electron density (or magnetic field) suddenly increases across the entire length of the emission region. This could be interpreted as an increase in the density of the material being injected into the jet, or perhaps an increase in the density of the radiating material by way of a shock front passing through the jet.