The X-ray nuclei of intermediate-redshift radio sources

We present a Chandra and XMM-Newton spectral analysis of the nuclei of the radio galaxies and radio-loud quasars from the 3CRR sample in the redshift range 0.1 < z < 0.5. In the range of radio luminosity sampled by these objects, mostly Fanaroff-Riley type IIs (FR IIs), it has been clear for some time that a population of radio galaxies [low-excitation radio galaxies (LERGs)] cannot easily participate in models that unify narrow-line radio galaxies and broad-line objects. We show that low-excitation and narrow-line radio galaxies have systematically different nuclear X-ray properties: while narrow-line radio galaxies universally show a heavily absorbed nuclear X-ray component, such a heavily absorbed component is rarely found in sources classed as low-excitation objects. Combining our data with the results of our earlier work on the z < 0.1 3CRR sources, we discuss the implications of this result for unified models, for the origins of mid-infrared emission from radio sources, and for the nature of the apparent Fanaroff-Riley type I (FR I)/FR II dichotomy in the X-ray. The lack of direct evidence for accretion-related X-ray emission in FR II LERGs leads us to argue that there is a strong possibility that some, or most, FR II LERGs accrete in a radiatively inefficient mode. However, our results are also consistent with a model in which the accretion mode is the same for low- and high-excitation FR IIs, with the lower accretion luminosities in FR II LERGs attributed instead to more efficient radio luminosity production in those objects.