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Recent investigations have shown that many parameters and assumptions made inthe application of spectral ageing models to FR-II radio galaxies (e.g.injection index, uniform magnetic field, non-negligible cross-lobe agevariations) may not be as reliable as previously thought. In this paper we usenew VLA observations, which allow spectral curvature at GHz frequencies to bedetermined in much greater detail than has previously been possible, toinvestigate two cluster-centre radio galaxies, 3C438 and 3C28. We find that forboth sources the injection index is much steeper than the values traditionallyassumed, consistent with our previous findings. We suggest that the Tribblemodel of spectral ageing provides the most convincing description when bothgoodness-of-fit and physically plausibility are considered, but show that evenwith greatly improved coverage at GHz frequencies, a disparity exists incluster-centre FR-IIs when spectral ages are compared to those determined froma dynamical viewpoint. We find for 3C438 that although the observationsindicate the lobes are expanding, its energetics suggest that the radiatingparticles and magnetic field at equipartition cannot provide the necessarypressure to support the lobes, similar to other cluster-centre source such asCygnus A. We confirm that small scale, cross-lobe age variations are likely tobe common in FR-II sources and should be properly accounted for whenundertaking spectral ageing studies. Contrary to the assumption of someprevious studies, we also show that 3C28 is an FR-II (rather than FR-I) source,and suggest that it is most likely a relic system with the central engine beingturned off between 6 and 9 Myrs ago.

Spectral ageing in the lobes of cluster-centre FR-II radio galaxies