Bow Ties in the Sky II: Searching for Gamma-ray Halos in the Fermi Sky
  Using Anisotropy
  Many-degree-scale gamma-ray halos are expected to surround extragalactic
high-energy gamma ray sources. These arise from the inverse Compton emission of
an intergalactic population of relativistic electron/positron pairs generated
by the annihilation of >100 GeV gamma rays on the extragalactic background
light. These are typically anisotropic due to the jetted structure from which
they originate or the presence of intergalactic magnetic fields. Here we
propose a novel method for detecting these inverse-Compton gamma-ray halos
based upon this anisotropic structure. Specifically, we show that by stacking
suitably defined angular power spectra instead of images it is possible to
robustly detect gamma-ray halos with existing Fermi Large Area Telescope (LAT)
observations for a broad class of intergalactic magnetic fields. Importantly,
these are largely insensitive to systematic uncertainties within the LAT
instrumental response or associated with contaminating astronomical sources.
