Intergalactic magnetic field spectra from diffuse gamma rays
  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. 
Wenlei Chen, Borun D. Chowdhury, Francesc Ferrer, Hiroyuki Tashiro and   Tanmay Vachaspati
astro-ph.CO astro-ph.HE hep-ph hep-th