Radio jet kinematics and γ-ray emission
While most of the photons emitted from radio jets are picked up at radio wavelengths, there is a very energetic emission component originating within AGN radio jets and is observed at X-ray and γ-ray wavelengths. Already EGRET, the γ-ray space mission found a clear association between radio-loud AGN and γ-ray emission. The Fermi space telescope confirmed this link, revealing thousands of radio-loud AGN to be γ-ray emitters. γ-rays are thought to be produced through the process of inverse Compton scattering of incident photons from the relativistic electrons within the radio jets. Different models prescribed seed photons from either the jet itself (synchrotron self-Compton) or from external photon fields (such as the accretion disk or the cosmic wave background). The actual site of the γ-ray emission production remains unclear, although several pieces of evidence point towards the very inner part of the radio jets.
Using the first year Fermi-LAT catalog and a statisticaly complete sample of radio-loud AGN we looked for a claimed link between large Doppler factors (apparent velocities) and γ-ray emission. Our analysis did not reveal such a connection. On the contrary, we found a sizeable fraction of γ-ray emitting AGN to have mildly super-luminal or even sub-luminal jet expansions speeds. Together with previous literature results, we interpreted our findings as evidence for a spine-sheath jet structure, with the highly relativistic spine being responsibly for the γ-ray emission from radio jets.
Related papers: Karouzos et al. 2011
Top: Apparent speed distributions for γ-ray detected (black histogram) and non-detected (gray shaded histogram) AGN. Bottom: Max apparent velocity as a function of γ-ray luminosity for γ-ray variable and non-variable sources. Figures from Karouzos et al. (2011).
