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.