Evolution of the power-law index Γ_PL of each spectrum (Clavel et al. 2016) as a function of its luminosity in the 1−10 keV range. All four outbursts during the 2000s are overplotted (color-coding as in Fig. 9). Bottom panel: zoom-in on the zone inside the orange dashed rectangle in the top panel. For clarity, we do not show observations when Γ_PL is unconstrained (soft states), but the usual values lie in the Γ_PL ≈ 2.0−3.0 range.