Scan of the cosmic-ray density profile power-law index α with respect to the data in Fig. 4 and earlier measurements from H.E.S.S. Collaboration (2016). The vertical dashed lines mark the derived 1 (blue), 2 (orange) and 3σ (red) confidence intervals. The horizontal dashed line represent the number of degrees of freedom in the fit. The scanned value, α corresponds to the true 3D cosmic-ray distribution and is projected on the plane of the sky using an assumed 3D gas map. See Sect. 4.2 for details.