Evolution of ν₀ in lag-frequency spectra as a function of spectral hardness. Left panel: results from the rise, plateau, and bright decline phases, which are observed to follow the same trend. The dotted black line is the best-fit model. Right panel: results from the hard-soft transition phase as compared to the other phases (grey symbols), which is observed to break from the observed trend at higher spectral hardness.The arrows are 90 per cent lower and upper limits.