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Intergalactic magnetic fields (IGMF) can cause the appearance of halos aroundthe gamma-ray images of distant objects because an electromagnetic cascadeinitiated by a high-energy gamma-ray interaction with the photon background isbroadened by magnetic deflections. We report evidence of such gamma-ray halosin the stacked images of the 170 brightest active galactic nuclei (AGN) in the11-month source catalog of the Fermi Gamma-Ray Space Telescope. Excess overpoint spread function in the surface brightness profile is statisticallysignificant at 3.5\sigma (99.95% confidence level), for the nearby, hardpopulation of AGN. The halo size and brightness are consistent with IGMF,B_{IGMF} ~ 10^{-15} G. The knowledge of IGMF will facilitate the futuregamma-ray and charged-particle astronomy. Furthermore, since IGMF are likely tooriginate from the primordial seed fields created shortly after the Big Bang,this potentially opens a new window on the origin of cosmological magneticfields, inflation, and the phase transitions in the early Universe.

Evidence for Gamma-Ray Halos Around Active Galactic Nuclei and the First Measurement of Intergalactic Magnetic Fields