Gamma-ray time variability studies are powerful tools for constraining the dimensions of the regions where high-energy emission occurs. This analysis is particularly useful for AGN with jets not directly pointing towards the observer (Misaligned AGN; MAGN). Indeed, for GeV-emitting AGN, the origin of the high-energy photons is still unclear. They could be produced in the compact core, in knots along the jet, or in the extended radio lobes.
In the nearby radio galaxy Centaurus A, the gamma-ray flux in the lobes and core are roughly the same, and in the Fanaroff-Riley (FR) type I radio galaxy M 87, multiwavelength campaigns seem to indicate multiple emitting regions in the jet. Finally, a recent variability study of the FR type II radio galaxy 3C 111 has excluded a significant GeV contribution from radio lobes and/ or hotspots and localized the dissipation region, at least during a flare, to a distance of < 0.3 pc from the black hole.
In order to further explore this issue, we began a systematic gamma-ray variability study of all the MAGN listed in the published LAT catalogs with the addition of 3C 120 and Pictor A. Four years of LAT data are analyzed using different time bin intervals in order to constrain a minimal time scale of variability for each source. On the basis of simple causality arguments, an estimation of the maximal dimension of the high-energy emitting regions is provided. Finally, we attempt to quantify the fraction of time that MAGN spend in the flare state in order to determine if the paucity of MAGN emitting in the GeV range could be due to variability effects.