Photon index, Γ, vs. the QPO frequency, ν_L, for MAXI J1348–630 (target source) and XTE J1550–564 (reference source). We use a specific function f(ν), detailed in Eq. (3) of Sect. 3.2.1, to fit the data, which gives us the corresponding solid lines. We can see that Γ is correlated with ν_L and saturated for ν_L > 6.8 Hz and ν_L > 10 Hz for MAXI J1348–630 and XTE J1550–564, respectively. Data for XTE J1550–564 were taken from ST09.