The radio colour-colour diagram of Van der Laan bubbles - an application to SS433

A radio “colour-colour” diagram is defined in order to determine the evolutionary state of synchrotron-emitting plasma bubbles that are ejected during outbursts in microquasars. We establish the colour-colour diagram for the plasmons of SS433 observed on 18 April 1998 using VLBI observations. We show that the radio plasmons are not consistent with a simple expanding sphere of plasmon. This may indicate that in-situ particle acceleration is taking place away from the central engine.