Fermi Gamma-ray Space Telescope

On the Location of the 2009 GeV Flares of Blazar PKS 1510-089

A. Dotson
E. T. Meyer, K. McCann

Abstract:

Most of the blazar emission is produced at GeV energies via inverse Compton scattering at an unknown distance from the black hole. Possible seed photon sources corresponf to different locations in the jet ranging from the broad-line region (BLR, ?0.1 pc), to the molecular torus (MT, ? 1- 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 in blazar PKS 1510?089 using four bright gamma-ray flares detected by Fermi in 2009. As shown by Dotson et al. (2012), 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 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 emission sites are spread over a wide range of locations beyond the BLR.