Correlation between the softness ratio (the ratio of count rates between 0.3−2 and 2−10 keV bands), and 0.3−10 keV count rate, for Obs2 to Obs5 (left panel), and for Obs1 and Obs6 (right panel). In both left and right panels, the black solid line is the best-fit linear model of y = (0.06 ± 0.01)x + (1.54 ± 0.13) for Obs2 to Obs5. The shaded grey region is the associated 1σ uncertainty.