The Accretion Geometry in Radio-Loud Active Galaxies

TL;DR

This paper reviews the current understanding of the accretion geometries in radio-loud AGN, highlighting differences between low and high luminosity sources and the conditions necessary for jet formation.

Contribution

It synthesizes recent multiwavelength observations to compare accretion flows in radio-loud AGN with their radio-quiet counterparts, proposing conditions for jet formation.

Findings

Low-luminosity AGN likely have thick, radiatively inefficient accretion flows.

Higher luminosity broad-line radio galaxies show evidence of thin disks close to the black hole.

Jet formation requires a rapidly spinning black hole, specific accretion flow geometry, and magnetic field conditions.

Abstract

We review the latest attempts to determine the accretion geometry in radio-loud active galactic nuclei (AGN). These objects, which comprise ~10-20% of the AGN population, produce powerful collimated radio jets that can extend thousands of parsecs from the center of the host galaxy. Recent multiwavelength surveys have shown that radio-loudness is more common in low-luminosity AGN than in higher luminosity Seyfert galaxies or quasars. These low-luminosity AGN have small enough accretion rates that they are most likely accreting via a geometrically thick and radiatively inefficient accretion flow. In contrast, X-ray spectroscopic observations of three higher luminosity broad-line radio galaxies (3C 120, 4C+74.26 and PG 1425+267) have found evidence for an untruncated thin disk extending very close to the black hole. These tentative detections indicate that, for this class of radio-loud AGN, the accretion geometry is very similar to their radio-quiet counterparts. These observations suggest that there are three conditions to jet formation that must be satisfied: the presence of a rapidly spinning black hole, an accretion flow with a large H/r ratio, and a favorable magnetic field geometry.