Meeting the Challenge from Bright and Fast Gamma-Ray Flares of 3C 279

Abstract

Bright and fast gamma-ray flares with hard spectra have been recently detected from the blazar 3C 279, with apparent GeV luminosities up to 10<SUP>49</SUP> erg s<SUP>-1</SUP>. The source is observed to flicker on timescales of minutes with no comparable optical-UV counterparts. Such observations challenge current models of high-energy emissions from 3C 279 and similar blazar sources that are dominated by relativistic jets along our line of sight with bulk Lorentz factors up to Γ ∼ 20 launched by supermassive black holes. We compute and discuss a model based on a clumpy jet comprising strings of compact plasmoids as indicated by radio observations. We follow the path of the synchrotron radiations emitted in the optical-UV bands by relativistic electrons accelerated around the plasmoids to isotropic Lorentz factors γ ∼ {10}<SUP>3</SUP>. These primary emissions are partly reflected back by a leading member in the string that acts as a moving mirror for the approaching companions. Around the plasmoids, shrinking gap transient overdensities of seed photons build up. These are upscattered into the GeV range by inverse Compton interactions with the relativistic electrons accelerated in situ. We show that such a combined process produces bright gamma-ray flares with minor optical to X-ray enhancements. Main features of our model include: bright gamma-ray flares with risetimes as short as a few minutes, occurring at distances of order 10<SUP>18</SUP> cm from the central black hole; Compton dominance at GeV energies by factors up to some 10<SUP>2</SUP>; minimal reabsorption from local photon-photon interactions.