PSF photometry (red), proper image subtraction (blue), and mean (black) time lags as a function of wavelength. All lags are measured with respect to the 7320 Å band. The dashed lines show the best fit to the observed relation τ = R_λ0/c [(λ/λ₀)^β − 1] with R_λ0/c and β as free parameters. The dotted lines are fits with fixed theoretical power-law index β = 4/3, as expected for an optically thick and geometrically thin disk. The shaded envelope shows the results of the JAVELIN Thin Disk inspired model.