LOWER LIMIT ON THE STRENGTH AND FILLING FACTOR OF EXTRAGALACTIC MAGNETIC FIELDS

Abstract

High energy photons from blazars can initiate electromagnetic pair cascades\ninteracting with the extragalactic photon background. The charged component of\nsuch cascades is deflected and delayed by extragalactic magnetic fields (EGMF),\nreducing thereby the observed point-like flux and leading potentially to multi\ndegree images in the GeV energy range. We calculate the fluence of 1ES 0229+200\nas seen by Fermi-LAT for different EGMF profiles using a Monte Carlo simulation\nfor the cascade development. The non-observation of 1ES 0229+200 by Fermi-LAT\nsuggests that the EGMF fills at least 60% of space with fields stronger than\n{\\cal O}(10^{-16}-10^{-15})G for life times of TeV activity of {\\cal\nO}(10^2-10^4)yr. Thus the (non-) observation of GeV extensions around TeV\nblazars probes the EGMF in voids and puts strong constraints on the origin of\nEGMFs: Either EGMFs were generated in a space filling manner (e.g.\nprimordially) or EGMFs produced locally (e.g. by galaxies) have to be\nefficiently transported to fill a significant volume fraction, as e.g. by\ngalactic outflows.\n