On the Magnetic Field in Quasar and FR II Large-Scale Jets

Here we report systematic comparison of the spectral properties of large-scale jets, hotspots and extended lobes in quasars and FR II radio galaxies recently observed with Chandra and ASCA. We argue that if the strong X-ray emission of the jet knots in these objects results from comptonisation of the CMB photons, as usually considered, the powerful large-scale jets are most likely far from the minimum-power condition in the sense that the magnetic field thereby is below equipartition. We also show that the X-ray emission of the hot-spots and lobes in the compiled dataset agrees with the minimum-power condition. In this context, we point out the need for substantial amplification of the magnetic field within the terminal shocks of powerful large-scale jets of quasars and FR II sources.