Cyclotron line centroid energy E_cyc as a function of the logarithm of flux in the 13−80 keV range. The black dots are the results of Klochkov et al. (2012), obtained with OSA 9 and corrected for the ISGRI energy gain drift. The red triangles are the results of this work, obtained with OSA 10 and then gain-corrected. The dashed and dotted lines are the results of linear fits to the E_cyc − log ₁₀ (Flux) data for the two sets resulting from OSA 9 and OSA 10 data reduction, respectively. The error bars indicate 1σ-uncertainties (the statistical flux uncertainties are smaller than the symbol size). The top x-axis shows the corresponding isotropic source luminosity assuming a distance of 2.4 kpc.