Lower limit on the strength and filling factor of extragalactic magnetic fields

Understand

High energy photons from blazars can initiate electromagnetic pair cascades interacting with the extragalactic photon background.

• The charged component of such cascades is deflected and delayed by extragalactic magnetic fields (EGMF), reducing thereby the observed point-like flux and leading potentially to multi degree images in the GeV energy range.
• We calculate the fluence of 1ES 0229+200 as seen by Fermi-LAT for different EGMF profiles using a Monte Carlo simulation for the cascade development.
• The non-observation of 1ES 0229+200 by Fermi-LAT suggests that the EGMF fills at least 60% of space with fields stronger than {\cal O}(10^{-16}-10^{-15})G for life times of TeV activity of {\cal O}(10^2-10^4)yr.