Histogram of deviations of the low‐energy power‐law indices from ﹩-2/ 3﹩ in units of their ﹩1\,\sigma ﹩ error for 3957 fitted spectra. Positive deviations represent a violation of the SSM. Also plotted is the distribution of ﹩1\,\sigma ﹩ deviations from the mean for spectra indices obtained from fits to 4000 simulated spectra (dotted). The peak has been clipped so that the wings of the two distributions can be compared.