Mean time lags (black circles) between multiband continuum light curves as a function of wavelength. All lags are measured relative to variations at 4250 Å. The dashed blue line shows the best fit to the observed relation τ = τ₀/ [(λ/λ₀) ^β − y₀], with τ₀, β, and y₀ as free parameters (these values are presented in Table 5). The red dotted line is a fit with fixed theoretical power-law index β = 4/3, as expected for an optically thick and geometrically thin disk.