Definition of source and background regions used to probe variations of the photon index across the remnant (top, the image is in the 0.4–10 keV band, adaptively smoothed and corrected for exposure and quiescent particle background) and the best-fit values of the photon index for each region (bottom, 1σ uncertainties). The dashed line represents the mean value and the shaded area corresponds to the photon index estimated using entire shell. For consistency, the region names correspond to the larger regions introduced in Fig. 2.