X-Ray Nuclei in Radio Galaxies: Exploring the Roles of Hot and Cold Gas Accretion

We present results from Chandra and XMM-Newton spectroscopic observations of the nuclei of z < 0.5 radio galaxies and quasars from the 3CRR catalog, and examine in detail the dichotomy in the properties of lowand high-excitation radio galaxies. The X-ray spectra of low-excitation sources (those with weak or absent optical emission lines) are dominated by unabsorbed emission from a parsec-scale jet, with no contribution from accretion-related emission. These sources show no evidence for an obscuring torus, and are likely to accrete in a radiatively inefficient manner. High-excitation sources (those with prominent optical emission lines), on the other hand, show a significant contribution from a radiatively efficient accretion disk, which is heavily absorbed in the X-ray when they are oriented close to edge-on with respect to the observer. However, the low-excitation/high-excitation division does not correspond to the FRI/FRII division: thus the Fanaroff-Riley dichotomy remains a consequence of the interaction between the jet and the hot-gas environment through which it propagates. Finally, we suggest that accretion of the hot phase of the IGM is sufficient to power all low-excitation radio sources, while high-excitation sources require an additional contribution from cold gas that in turn forms the cold disk and torus. This model explains a number of properties of the radio-loud active galaxy population, and has important implications for AGN feedback mechanisms. 1. Unified models and radio-loud AGN: the excitation dichotomy In standard AGN models, efficient disk accretion of cold matter on to the central supermassive black hole provides the radiation field that photoionizes the optical broad-line region (BLR) and narrow-line region (NLR) and gives rise to X-ray emission via Compton scattering. Without radiatively efficient accretion via the disk, none of these standard features of such an AGN would be observed. Unified models propose that a direct view of the BLR and the optical continuum may be obscured (e.g., in Seyfert 2s) by a dusty ‘torus’: but in this case the torus re-radiates strongly in the mid-IR band, so that the presence of a luminous AGN can still be inferred. By analogy with radio-quiet objects, we would expect that face-on radioloud objects (the broad-line radio galaxies and radio-loud quasars) would show both broad and narrow optical lines, while edge-on radio-loud objects (narrowline radio galaxies, NLRG) would show only narrow optical lines, and would Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 School of Physics, Astronomy & Mathematics, University of Hertfordshire, College Lane, Hat-