How is the blazar GeV emission really produced ?

We show that the external Compton (EC) model for the production of the GeV emission in blazars makes specific predictions for the spectrum and variability of those blazars characterized by a high Compton dominance (Compton to synchrotron luminosity ratio). These unavoidable features have not been observed, casting doubt on the validity of this popular model. We argue that synchrotron-self Compton (SSC) models including the higher orders of Compton scattering are more promising, and we briefly discuss the implications of our findings for the geometry of the broad line region (BLR). Introduction. The GeV emission from the relativistic jets of blazars is believed to be due to energetic electrons that inverse-Compton (IC) scatter to γ-ray energies lower energy photons. These photons can be synchrotron photons produced in the source (synchrotron-self Compton, SSC; e.g. Maraschi et al. 1993) and/or external photons such as the BLR UV photons (Sikora et al. 1994) which, as reverberation mapping shows, are produced at distances of ∼ 10 17−18 cm from the central engine (Kaspi et al. 2000). The second case is believed to be favored, because variability arguments show that the site of the blazar emission is at a comparable distance, and, therefore, it is exposed to the BLR photon field, which has a photon energy density in the jet frame boosted by Γ 2 , where Γ ∼ 10 is the Lorentz factor of the jet bulk motion.