How MOS1 (top panel) and MOS2 (bottom panel) vary with respect to pn. α ratios for MOS1/pn (red) and for MOS2/pn (blue) are plotted against energy (bold points). The data have been adaptively binned to follow the statistics. The faint points, connected by a thin line, show the results obtained using the “fit and stack” (see Sect. 4) analysis (the last MOS2/pn point is at α ≈ 0.75).