Upper panels: DCF between each pair of images, in the r band. The red line represents the mean correlation of unrelated quasars (from simulations). The solid curves represent the percentiles of that distribution: fuchsia (99), orange (95), green (5), and light blue (1). These curves define the range of correlations expected from random, unrelated light curves. The dashed line is the actual cross-correlation between a real pair of light curves; it approaches the upper percentile around a specific time delay, indicating a significant correlation that is unlikely to happen by chance. Lower panels: Distribution of centroid positions obtained from the bootstrap resampling, reflecting the uncertainty on the centroid determination.