What Kinds of Accretion Disks Are There in the Nuclei of Radio Galaxies?

Abstract

It seems to be a widely accepted opinion that the types of accretion disks (or flows) generally realized in the nuclei of radio galaxies and in further lower mass-accretion rate nuclei are inner, hot, optically thin, radiatively inefficient accretion flows (RIAFs) surrounded by outer, cool, optically thick, standard-type accretion disks. However, observational evidence for the existence of such outer cool disks in these nuclei is rather poor. Instead, recent observations sometime suggest the existence of inner cool disks of the non-standard type, which develop in a region very close to their central black holes. Taking NGC 4261 as a typical example of such light-eating nuclei, for which both flux data ranging from radio to X-ray and data for the counterjet occultation are available, we examine the plausibility of such a picture for the accretion states, as mentioned above, based on model predictions. It is shown that the explanation of the gap seen in the counterjet emission in terms of the free–free absorption by an outer standard disk is unrealistic and, moreover, the existence itself of such an outer standard disk seems to be very implausible. Instead, the model of RIAF in an ordered magnetic field (so-called resistive RIAF model) can well serve to explain the emission gap in terms of the synchrotron absorption, as well as to reproduce the observed features of the overall spectral energy distribution (SED). This model also predicts that the RIAF state starts directly from an interstellar hot gas phase at around the Bondi radius, and terminates at the inner edge, whose radius is about 100-times the Schwartzschild radii. Therefore, there is a good possibility for a cool disk to develop within this innermost region.