Mrk 509 variance spectrum (i.e. the variance in units of (mJy)² plotted as a function of photon frequency). Black points show the observed variance measurements calculated from light curve segments of duration, T_max = 12 days (listed in Table 2). Open circles indicate data points excluded during the model fits. The black dashed line shows the best linear fit (in log-log space) to the data. The best-fit slope is b = 0.5 ± 0.4. The blue lines show the X-ray reverberation best-fit models for α^* = 0 (left panel) and α^* = 1 (right panel). The model’s M2 variance is lower than the adjacent W1 and W2 variances because of the ∼2200 Å feature in the Milky Way’s extinction law as determined by Cardelli et al. (1989). The bottom panels show the best-fit residuals, i.e. the observed variance and best fit model variance over the observed variance error, in the case of the X-ray reverberation best-fit models (blue lines). Grey squares on the top show the measured excess variance calculated using the full light curves, while the solid black line shows the best-fit model prediction for these measurements (see Sect. 6.4 for more details).