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6.3. Radio-loud AGN

It has long been known that most AGN can be readily characterized as being radio-loud (i.e. with relativistic radio jets) or radio-quiet (i.e. with no well-defined radio jets). One of the greatest mysteries in the field of AGN research is the physical mechanism underlying this division. A first step in solving this puzzle is to compare and contrast the central engine structures of radio-quiet and radio-loud AGN. Since they originate from deep within the central engine, X-rays are a good tool for probing any such differences.

Radio-loud AGN are rarer, and hence typically fainter, that 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 cluster emission. For these reasons, the quality of te observational constraints is rather poorer in the case of radio-loud AGN as compared with radio-quiet sources. Having stated those caveats, there does appear to be a difference between the X-ray 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 (Eracleous, Halpern & Livio 1996; Wozniak et al. 1998; Reynolds et al. 1997; Sambruna, Eracleous & Mushotzky 1999; Grandi et al 1999; Eracleous et al 2000). 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 a physical state incapable of producing reflection signatures (such as an ADAF or some similarly hot state). Future observations with XMM-Newton should be able to distinguish these possibilities by searching for very weak broad components to the iron line.