Ratios of the cyclotron line energies E_n/E₀ with respect to the centroid energy of the fundamental line against time. The dark green, dark blue, red and purple data points (bars) correspond to E₁/E₀, E₂/E₀, E₃/E₀, and E₄/E₀, respectively, using the CutoffPL continuum with a 10 keV feature. The light green and cyan data points (filled circles) show the results for E₁/E₀ and E₂/E₀ using NPEX. The dashed lines indicate integer values for these ratios.