Determination of intergalactic magnetic fields from gamma ray data

► Intergalactic magnetic fields are between 0.01 and 30 femtogauss. ► The lower limit is independent of cosmic-ray production rate and spectrum. ► Data provide evidence of cosmic-ray production in active galactic nuclei. We report a measurement of intergalactic magnetic fields using combined data from Atmospheric Cherenkov Telescopes and Fermi Gamma-Ray Space Telescope, based on the spectral data alone. If blazars are assumed to produce both gamma rays and cosmic rays, the observed spectra are not sensitive to the intrinsic spectrum of the source, because, for a distant blazar, secondary photons produced along the line of sight dominate the signal. In this case, we set a limit 1 × 10 −17 G < B < 3 × 10 −14 G. If one excludes the cosmic-ray component, the 10 −17 G lower limit remains, but the upper limit depends on the spectral properties of the source. We present the allowed ranges for a variety of model parameters.