Cyclotron line centroid energy E_cyc as a function of the logarithm of flux in the 4−80 keV range. The error bars indicate 1σ-uncertainties (the flux uncertainties are smaller than the symbol size). The top X-axis represents the corresponding isotropic source luminosity assuming a distance of 2.4 kpc. The dotted line shows a linear fit to the E_cyc–log₁₀(Flux) dependence.