spectrum of white and red noise random processes (broken lines). One can see that the 3.3 ks detection exceeds the expected levels of white and red noise at the 99$\%$ significance level, i.e. the probability of the detection is higher than 99$\%$ of the noise random processes (the significance of this signal is 99.979$\%$ relative to red noise with $\alpha = 0.14$). The origins of the white and red noise power spectra were discussed in Section \[sigtestsection\].

The cross-wavelet analysis for 3C 273 (Fig. \[3ccross\]) supports the conclusion that a period of  3.3 ks is indeed present. Here, the XWT (see Section \[xwtsection\]) compares a mock sinusoidal signal with a period of 3282 s with the 3C 273 light curve. The concentration in the cross-wavelet transform shows that the 3.3 ks signal is present throughout the observation. As one can see, by comparing the crosswavelet signals in juxtaposed bands, the 3.3 ks periodicity can be traced over the entire interval. In the CWT (Figure \[3ccwt\]) the 3.3 ks signal is particularly strong at late times, and so, due to the limited dynamic range of the rendering, is not evident early in the time interval in that figure.

This periodicity is 