Iron lines from radio-loud AGN : a clue to the dichotomy?

One of the greatest mysteries in the field of AGN research is the physical mechanism underlying the radio-quiet/radio-loud dichotomy. Why do some black hole systems choose to launch, accelerate and collimate powerful relativistic jets, whereas other systems do not? The natural environment in which to form an energetic relativistic jet is the relativistic region close to an accreting black hole. A first step in addressing this problem from an observational standpoint is to compare and contrast the central engine structures of radio-quiet and radio-loud AGN. Since they are direct probes of the inner accretion disk, broad iron lines provide a powerful way of facilitating such a comparison.

Radio-loud AGN are rarer, and hence typically fainter, than their radio-quiet counterparts. Furthermore, many of the best candidates for study are found in clusters of galaxies and it can be difficult to observationally distinguish AGN emission from thermal X-rays emitted by $10^7-10^8{\rm\thinspace K}$ gas trapped in the cluster's gravitational potential well. For these reasons, the quality of the observational constraints are rather poorer than for Seyfert-1 galaxies. There does, however, appear to be a difference between the iron line properties of radio-loud nuclei and radio-quiet nuclei. Broad iron lines, and the associated Compton reflection continua, are generally weak or absent in the radio-loud counterparts [257,258,259,260,261,262,263]. This effect might be due to the swamping of a normal `Seyfert-like' X-ray spectrum by a beamed jet component (similar to the swamping of optical emission lines in a blazar spectrum). Alternatively, the inner disk might be in the form of a very hot and optically-thin radiatively-inefficient accretion flow. Finally, the inner disk might be radiatively-efficient and optically-thick, but not produce prominent X-ray reflection features due to a very high ionization [264]. Again, future observations with XMM-Newton should be able to distinguish these possibilities by searching for very weak broad components to the iron line and iron edges with high signal-to-noise.