Long term trend in the mean flux density and modulation index at 4.9 GHz. Determining the intrinsic flux density and modulation index of a scintillating source requires a measurement that extends over a large number of independent realisations of the scintillation process. Monte Carlo simulations (Dennett-Thorpe & de Bruyn 2003) suggest that the relative error in the flux density decreases as ≈ 1/3N^{− 1/2} where N is the number of peak-to-peak variations, or scintles, exhibited by the lightcurve. To minimize errors in the long-term lightcurve we therefore only include data taken in observations extending over at least 6 scintles. The flux density and modulation index observed in the 8 GHz VLBA observation on 23 Feb. 2007, were scaled to 4.9 GHz using the average WSRT values observed at those frequencies in the preceding 3 years. The 2009.0 data points are from Koay et al. (2011).