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

Abstract

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 213CR 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 (F 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 ^ P 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'.