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TeV BL Lacertae objects require extreme relativistic bulk motions in the gamma-ray emission region, but at the VLBI scale their radio knots hardly move. The same sources show evidence, in radio, of a structure made of a fast spine plus a slow layer. We propose that this structure exists even on the spatial scale of regions responsible for the gamma-ray emission. One component sees the (beamed) radiation produced by the other, and this enhances the inverse Compton emission of both components. In addition, this allows the magnetic field to be nearly in equipartition with the emitting particles. The inverse Compton emission of the spine is anisotropic in its frame, possibly producing a deceleration of the spine by the Compton rocket effect. In this scenario, the slow layer is also a relatively strong high-energy emitter, and thus radiogalaxies become potentially detectable by GLAST.
Ghisellini et al. (Tue,) studied this question.