X-ray observations of radio-loud AGN

One of the fundamental still open questions of extragalactic astronomy is the difference between radio-loud and radio-quiet AGN. While both types are thought to be ultimately powered by accretion onto a supermassive black hole, it is puzzling how radio-loud sources manage to produce powerful, relativistic jets which are instead absent in their radio-quiet counterparts. Recent studies show that the host galaxies and environments of the two types of AGN are not radically different as once thought, restricting the origin of the radio-loud/radio-quiet (RL/RQ) dichotomy to the structure of their central engines.

The X-rays may be the key to understand RL/RQ AGN since they are produced in the inner regions, and carry to us features which are the imprints of the accretion process. These include 1) the fluorescent Fe Kalpha emission line at rest-frame energies 6.4 keV, whose profile depends strictly on the gravitational potential well around the black hole and the size of the emission region; and 2) the Compton reflection component - a bump peaking around 20 keV in the X-ray spectrum. Studies of Seyfert 1s with ASCA and RXTE showed that both features are strong in these sources. In particular, the Fe Kalpha line profile is generally broad and asymmetric to the red, supporting an origin in a standard, cold accretion disk, from gas at a distance of 6-10 gravitational radii from the black hole.

Our group (R. Sambruna, M. Eracleous, and R. Mushotzky) was among the first to start a systematic study with ASCA and RXTE of radio-loud AGN, specifcally Broad-Line Radio Galaxies (BLRGs) and quasars. We found that the X-ray spectra of radio-loud AGN are different from their radio-quiet counterparts (Sambruna et al. 1999; Eracleous et al. 2000; see summary in Sambruna et al. 2002). Briefly, the Fe Kalpha line is narrower and weaker in BLRGs than in Seyfert 1s, with equivalent widths EW ~ 100 eV, widths ~ 40,000 km/s FWHM, and unresolved profiles. In contrast, Seyfert 1s typically have EWs ~250 eV, width ~50,000 km/s FWHM, and the line profile has a prominent red wing. In addition, the Compton reflection hump is weak or absent in BLRGs, with upper limits to its strength R~0.5, while in most Seyfert 1s R~1, where R=Omega/2pi is the covering factor of the reprocessing medium.

We interpret these results as evidence for a fundamentally different structure of the accretion flow in RL sources. One possibility is that the inner disk in BLRGs is occupied by an ion torus, or ADAF. In this scenario, the \feka\ line in BLRGs is narrower than in Seyferts because it is produced in the cold disk exterior to the ADAF, at larger distances from the black hole. An alternative model was recently proposed by Ross, Fabian, and collaborators, and purports that the disks in BLRGs are more strongly ionized than in Seyfert 1s, explaining the lack of strong Compton reflection components. It is also possible that the reflection features in BLRGs originate in a distant molecular torus, as proposed by other groups.

While there is no confirmed model, the bottom line from ASCA, RXTE, and SAX studies of RL AGN is that their X-ray spectra are fundamentally different from RQ AGN. We are in the process to acquire more sensitive XMM and Chandra data for well-selected RL sources in order to test in detail the structure of their central engines and discriminate among the above models.

This work is supported by NASA funds through the LTSA, ADP, and XMM programs.

Recent Publications of the Group:

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