SNR radio continuum spectral indices and ages. Panel a: histogram of the spectral indices as inferred from the weighted fitting to the entire spectrum of each SNR in our study (Fig. 3). Panel b: distribution of the computed spectral index values according to the age and morphology of the source. The spectral indices for the younger (≲3000 yr) SNRs are steeper than −0.5, the canonical value in test-particle diffusive shock acceleration theory.