The Spectra and Energies of Classical Double Radio Lobes

We compare two temporal properties of classical double radio sources: i)radiative lifetimes of synchrotron-emitting particles and ii) dynamical sourceages. We discuss how these can be quite discrepant from one another, renderinguse of the traditional spectral ageing method inappropriate: we contend thatspectral ages give meaningful estimates of dynamical ages only when these agesare << 10^7 years. In juxtaposing the fleeting radiative lifetimes with sourceages which are significantly longer, a refinement of the paradigm for radiosource evolution is required. The changing spectra along lobes are explained,not predominantly by synchrotron ageing but, by gentle gradients in a magneticfield mediated by a low-gamma matrix which illuminates an energy-distributionof particles, controlled largely by classical synchrotron loss in the highmagnetic field of the hotspot. The energy in the particles is an order ofmagnitude higher than that inferred from the minimum-energy estimate, implyingthat the jet-power is of the same order as the accretion luminosity produced bythe quasar central engine. This refined paradigm points to a resolution of thefindings of Rudnick et al (1994) and Katz-Stone & Rudnick (1994) that both theJaffe-Perola and Kardashev-Pacholczyk model spectra are invariably poordescriptions of the curved spectral shape of lobe emission, and indeed that forCygnus A all regions of the lobes are characterised by a `universal spectrum'.[abridged]Comment: LaTeX, 4 figures. To appear in A