Combined lag–spectrum. Red circular points show our HALO measurements, while black square points indicate reanalyzed literature measurements based on the light curve data from Edelson et al. (2024). The horizontal error bars represent the rms width of the transmission curve as the filter’s wavelength uncertainty. The dot-dashed red and black lines, together with their shaded regions, represent the best-fit results from power-law lag–spectrum fitting, with β and α treated as free parameters and fixed at 4/3 and 1, respectively. The corresponding fits obtained excluding τ_uI from the lag-spectrum are shown by the dashed blue and maroon lines. The lag–spectrum shows a clear excess in the u and U bands, suggesting Balmer continuum contamination from the BLR in these bands.