The nature of the low energy turnover in powerful FRII sources

The direct detection of the lowenergy turnover in the eastern hotspot of6C0905+3955 puts serious constraints on the models of the acceleration process.To detect similar turnovers, and investigate their dependence on luminosity, wehave chosen a sample of large, powerful, distant, FRII radio sources thatcontain huge reservoirs of aged synchrotron plasma, emitting Xrays viainverseCompton scattering of CMB photons. We aim to exploit XMM.s excellentsoft response and sensitivity to detect this emission and in doing so expand notonly our sample of sources with putative lowenergy turnovers in their hotspotsbut also measure the energy contained in the low Lorentzfactor electrons inthese sources, which has important implications for the energy dumped into any group or cluster medium. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]