  Non-vanishing parity-odd correlators of gamma ray arrival directions observed
by Fermi-LAT indicate the existence of a helical intergalactic magnetic field
(Tashiro et al.2013). We successfully test this hypothesis using more stringent
cuts of the data, Monte Carlo simulations with Fermi-LAT time exposure
information, separate analyses for the northern and southern galactic
hemispheres, and confirm predictions made in Tashiro & Vachaspati (2014). With
some further technical assumptions, we show how to reconstruct the magnetic
helicity spectrum from the parity-odd correlators.
