tokens: versions of these methods as described in Patiño-Álvarez et al. ( 2013 ). To establish a significance to the correlation coefficients we performed Monte carlo simulations following Emmanoulopoulos, McHardy Papadakis ( 2013 ) and Patiño-Álvarez et al. ( 2018 ). The confidence intervals (at 90 per cent) in the delay were obtained as in Patiño-Álvarez et al. ( 2018 ). To determine the truthfulness of the delays obtained with the cross-correlation analysis, we performed an alias check via Fourier analysis (Press et al. 2007 ). 9 The resulting delays obtained from the cross-correlation analysis are summarized in Table 3 . Table 3. Delays obtained via cross-correlation analysis, for the entire time-range of study. Delays are in units of days. All correlations are at a confidence level , unless otherwise stated. The uncertainty is at a 90% confidence level. a All delays observed were found to be alias. b Inconclusive analysis. It is important to mention that there are non-conclusive results for some of the light curve pairs, where lag periods from 200 to 1000 d, appear as significant correlations (see Fig. 9 ). We explain this effect by taking into account the low-amplitude variability over large periods of the light curves. When the cross-correlation coefficient is calculated for every lag, light curves in quiescent states will be very similar to each other, regardless of the time axis-shift. The same issue also contaminates cross-correlation functions for correlated signals. Figure 9. Example of inconclusive cross-correlation result. The noisy lines represent the significance of correlation at 90, 95, and 99 per cent, below and above correlation coefficient zero. The smooth line is the interpolated cross-correlation function (ICCF). 5.1 <italic>V</italic> band and 15 GHz anticorrelation In Section 5 , we reported finding a weak anticorrelation between the V band and 15 GHz. We decided to investigate this further by analysing the normalized light curves, then we noticed that during the time period JD 245 = 4860–5760, the behaviour of the V band and the 15 GHz light curves is contrary (see Fig. 10 ). However, this anticorrelation does not hold for the entirety of the time-frame studied in this work; which is the likely reason why it only appeared as a weak anticorrelation on the analysis mentioned above. Figure 10. Normalized light curves for the V band and the 15 GHz emission in the period JD 245 = 4860–5760. Moreover, we decided to test it with a cross-correlation analysis, restricted to the period mentioned above. The analysis results in an anticorrelation at a delay of 39.6 ± 3.2 d, with the V band leading the 15 GHz emission (see Fig. 11 ). In blazars, the V band is usually dominated by synchrotron emission from the jet; however, 3C 273 has a history of being dominated by the accretion disc in the optical bands (e.g. Shields 1978 ; Ghisellini et al. 2010 ). This fact that the optical emission is mostly dominated by thermal emission from
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