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K. Blundell, S. Rawlings (Oxford University Astrophysics)
We consider two temporal properties of classical double radio sources and compare (i) radiative lifetimes of synchrotron-emitting particles with (ii) dynamical source ages. We discuss how these can be quite discrepant from one another, rendering use of the traditional spectral ageing method inappropriate: we contend that spectral ages give meaningful estimates of dynamical ages only when these ages are \ll 107 years. In juxtaposing the fleeting radiative lifetimes with source ages which are significantly longer, a refinement of the paradigm for radio source evolution is required. We move beyond the traditional bulk backflow picture and consider alternative means of the transport of high Lorentz factor (\gamma) particles, which are particularly relevant within the lobes of low luminosity classical double radio sources. The changing spectra along lobes are explained, not predominantly by synchrotron ageing but, by gentle gradients in a magnetic field frozen-in to a low-\gamma matrix which illuminates an energy-distribution of particles, N(\gamma), controlled largely by classical synchrotron loss in the high magnetic field of the hotspot. A model of magnetic field whose strength lowers with increasing distance from the hotspot, and in so doing becomes increasingly out of equipartition, is substantiated by constraints from different types of inverse-Compton scattered X-rays, and in turn narrows the uncertainty in the energy budgets of these objects. We describe the new constraints which spatially resolved, very-low-frequency imaging of these objects brings to these issues.
KMB thanks the Royal Society for a University Research Fellowship.
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