Evolution of the hard X-ray power law index Γ during the four outbursts of GX339-4. Left: hard X-ray power law index Γ as a function of the 3–9 keV X-ray luminosity of the pure hard state observations during the four GX339-4 outbursts (data from Clavel et al. 2016). In blue the rising phase and in red the decaying phase. Right: histograms of Γ. These distributions are subject to a certain number of observational biases: inclusion of error bars and the number of observations per phase.