Constraining the location of the emitting region in Fermi blazars through rapid γ-ray variability

We consider the 1.5-yr Fermi/Large Area Telescope (LAT) light curves (E > 100 MeV) of the flat spectrum radio quasars 3C 454.3 and PKS 1510-089, which show high activity in this period of time. We characterize the duty cycle of the source by comparing the time spent by the sources at different flux levels. We consider in detail the light curves covering periods of extreme flux. The large number of high-energy photons collected by LAT in these events allows us to find evidence of variability on time-scales of a few hours. We discuss the implications of significant variability on such short time-scales that challenge the scenario recently advanced in which the bulk of the gamma-ray luminosity is produced in regions of the jet at large distances (tens of parsec) from the black hole.