Evidence for Gamma-Ray Halos Around Active Galactic Nuclei and the First
  Measurement of Intergalactic Magnetic Fields
  Intergalactic magnetic fields (IGMF) can cause the appearance of halos around
the gamma-ray images of distant objects because an electromagnetic cascade
initiated by a high-energy gamma-ray interaction with the photon background is
broadened by magnetic deflections. We report evidence of such gamma-ray halos
in the stacked images of the 170 brightest active galactic nuclei (AGN) in the
11-month source catalog of the Fermi Gamma-Ray Space Telescope. Excess over
point spread function in the surface brightness profile is statistically
significant at 3.5\sigma (99.95% confidence level), for the nearby, hard
population of AGN. The halo size and brightness are consistent with IGMF,
B_{IGMF} ~ 10^{-15} G. The knowledge of IGMF will facilitate the future
gamma-ray and charged-particle astronomy. Furthermore, since IGMF are likely to
originate from the primordial seed fields created shortly after the Big Bang,
this potentially opens a new window on the origin of cosmological magnetic
fields, inflation, and the phase transitions in the early Universe.
