2015ApJ...809..164D

Most of the radiated power of blazars is produced at GeV energies via inverse Compton scattering at an unknown distance from the central engine. Possible seed photon sources map to different locations along the jet spanning two orders of magnitude in distance from the black hole, ranging from the broad-line region (BLR, ∼0.1 pc), to the molecular torus (MT, few pc), to the very long baseline interferometry (VLBI) radio core zone at ∼10 pc. Here, we apply a diagnostic for identifying the GeV emission zone (GEZ) in blazar PKS 1510-089 using four bright gamma-ray flares detected by Fermi in 2009. As shown by Dotson et al., the flare decay time should be energy-independent for flares in the BLR, but faster at higher energies for flares in the MT. We find that in the two cases where the gamma-ray flare was not accompanied by an optical flare, the decay times show an energy-dependence suggesting a location in the MT. For the two GeV flares accompanied by optical flares, we obtained very fast decay times (≲3 hr) in both low and high energy Fermi bands. For these flares, considering the simultaneous >100 GeV detection by HESS (H.E.S.S. Collaboration et al.) and the ejection of a superluminal component from the VLBI radio core in one case, our results suggest that both flares came from the vicinity of the VLBI core. We thus suggest that the GEZ is spread over a wide range of locations beyond the BLR.