Fermi Gamma-ray Space Telescope

Discerning the Origin of Orphan Gamma-Ray Flares

N. R. MacDonald
A. P. Marscher, S. G. Jorstad

Abstract:

Where do the highest-energy flares from within the jet of a blazar originate? It is commonly postulated that the GeV flares observed by the Fermi space telescope originate either from within the broad line region (BLR) of the host active galactic nuclei (AGN), or, instead, from within shocks many parsecs further downstream in the jet. Both of these locations contain seed photons that can be inverse-Compton scattered up to high-energies by relativistic electrons within the jet. The Fermi space telescope, however, has detected a sub-set of 'orphan' gamma-ray flares whose origin remains unknown. These flares seem to occur in relative isolation, with little or no variability detected at lower energies. MacDonald et al. (2015), in their 'Ring of Fire' model, suggest an alternative site of gamma-ray production situated between the BLR and the jet recollimation shock. MacDonald et al. propose that this alternative site is associated with a shocked portion of a more slowly moving sheath of plasma that enshrouds the relativistic spine of the jet. As a relativistic blob of plasma passes through this ring of shocked sheath, relativistic electrons within the blob will inverse-Compton scatter photons emanating from the ring, thus producing an orphan flare. This new model is able to reproduce a prominent orphan flare observed in the blazar PKS 1510-089 in 2009. I have since extended my modeling efforts to a larger sample of blazars that have all exhibited prominent orphan flares, and I will present the results of this new study. This work has been supported by NASA Fermi Guest Investigator grants NNX12AO79G and NNX14AQ58G.