Structure function SF(τ) of the α_OX index as a function of the time-lag τ for the multi-epoch objects in the reference sample. The dotted line is the noise level; the dashed line is the “uncorrected” structure function (i.e. without noise subtraction); the red line is the “corrected” SF, where red points are representative values of bins; the blue straight line is a weighted least-squares fit to the “corrected” SF. Error bars represent the 1-σ dispersion of the distribution of points in each bin.