Schematic diagram of the spectral index distributions without electron injection, assuming different synchrotron emission models. The blue line assumes the JP model, which is used in this study. The red line shows the spectral index distribution with the KP model. Assuming that the injection spectral index is α_inj = −0.7, the expected index after steepening is 4/3α_inj − 1 ≈ −1.933, which is within the allowable range obtained from observation.