Ageing and speeds in a representative sample of 21 classical double radio sources

Four-frequency observations with the VLA and 5-km telescope have been used to determine the spectral variations within the lobes of a representative sample of 21 3CR classical double radio sources. The variations in radio spectrum have been interpreted in terms of synchrotron-loss theory to deduce characteristic spectral ages for electrons within the lobes. On the assumption that acceleration occurs within the hotspots, mean speeds (v_L) for the relative rate of separation of hotspot and lobe material have been obtained for those sources with measured or estimated redshifts. It is found that: (i) no source has a value of v_L which violates the upper limit of 0.2c imposed by the statistics of hotspot separation asymmetries; (ii) v_L increases with radio power p_L , such that $${V}_{L}\,\propto\,{P}_\text{L}^{0.33}$$; (iii) acceleration of radiating electrons occurring everywhere within the lobes is not required, but may be occurring to a minor extent. This validates the initial assumption about acceleration being mainly confined to the hotspots; (iv) the sources in which spectral ageing is anomalous are found to have structures different from that of a typical ‘Classical Double’.