Increase of the CRL 618 flux density at 22 GHz (left panel) and 5 GHz (right panel) as a function of time. The dashed line is the result of a simultaneous power-law fit to the data of the flux at 22 GHz and 5 GHz, and the size of the source at 22 GHz (Fig. 2). The numerical expressions of the fits are also shown in the panels. We set the starting time of the ionization as a common parameter of the fit (see main text).