Constraints on the Intergalactic Magnetic Field from Bow Ties in the
  Gamma-ray Sky
  Pair creation on the cosmic infrared background and subsequent
inverse-Compton scattering on the CMB potentially reprocesses the TeV emission
of blazars into faint GeV halos with structures sensitive to intergalactic
magnetic fields (IGMF). Previous work has shown that these halos are then
highly-anisotropic and extended. If the coherence length of the IGMF is greater
than the inverse-Compton cooling length of the pairs, then the orientation of
the gamma-ray halo will be correlated with the direction of the magnetic field
which is unknown and expected to change for each source. In order to
constructively add each source we then use angular power spectra which are
insensitive to the jet orientation. By looking at known GeV blazars detected by
Fermi, we exclude the existence of an IGMF with coherence lengths $>100$ Mpc at
greater than $3.9\sigma$ with current-day strengths in the range $10^{-16}$ to
$10^{-15}$ G, and at 2$\sigma$ from $10^{-17}$ to $10^{-14}$ G. This provides a
direct measurement of the non-existence of gamma-ray halos, providing an
important check on previous results.
