Log-parabola photon index α versus the 3–7 keV flux from the Swift-XRT observations. The log-parabolic fits are performed after fixing the spectral curvature β = 0.16. The red cross represents the flare on MJD 57788 seen at VHE energies. The black line represent a linear fit, while the grey area (hardly visible in the plot) is its uncertainty. The slope of α versus the 3–7 keV flux is −0.64 ± 0.01. Hollow markers depict fits with a p-value below 5 × 10⁻², which are not considered in the linear fit.