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It has been suggested that relativistic shocks in extragalactic sources mayaccelerate the highest energy cosmic rays. The maximum energy to which cosmicrays can be accelerated depends on the structure of magnetic turbulence nearthe shock but recent theoretical advances indicate that relativistic shocks areprobably unable to accelerate particles to energies much larger than a PeV. Westudy the hotspots of powerful radiogalaxies, where electrons accelerated atthe termination shock emit synchrotron radiation. The turnover of thesynchrotron spectrum is typically observed between infrared and opticalfrequencies, indicating that the maximum energy of non-thermal electronsaccelerated at the shock is < TeV for a canonical magnetic field of ~100 microGauss. Based on theoretical considerations we show that this maximum energycannot be constrained by synchrotron losses as usually assumed, unless the jetdensity is unreasonably large and most of the jet upstream energy goes tonon-thermal particles. We test this result by considering a sample of hotspotsobserved with high spatial resolution at radio, infrared and opticalwavelengths.

Evidence that the maximum electron energy in hotspots of FR II galaxies is not determined by synchrotron cooling