small and of either sign, showing that there is no systematic curvature. The mean difference $\alpha_1 - \alpha_2$ across the entire data set is $-0.035$ compared to the rms fluctuation of $0.187$.

The observation that the relative flux density gradient is both greater and increasing more rapidly at the higher frequencies (Table \[tb:grad\]) implies a systematic change in the spectral index. Figure \[fg:si\_time\] shows the computed spectral indices from 843 MHz to 8.4 GHz as a function of time. Because of the uneven and non-simultaneous sampling at the different frequencies, spline curves were fitted to all data sets except that at 1.4 GHz. Spectral indices were then computed by interpolating values at 843 MHz, 2.4, 4.8 and 8.6 GHz to times when the 1.4 GHz flux density was measured, and fitting power-law spectra to flux densities. The interpolation was unreliable near the start of the data set for 2.4 and 8.6 GHz because of sparse data, so only three points were fitted there.

Apart from a few high points between days 1200 and 1400 near the start of the data set (which have large error bars), there is a more-or-less steady increase in spectral index, corresponding to a flattening of 