TeV photons coming from a distant extragalactic source are not able to propagate over large distances because they interact with the extragalactic background light photons producing pairs e+e−. These pairs, in turn, interact with the cosmic microwave background via inverse-compton producing a secondary γ-ray emission at lower energies. Extragalactic magnetic fields alter the trajectories of the electron pairs hence the secondary emission can be extended around the TeV sources. This process can be used to constrain the magnetic fields in the inter-galactic medium. We present the study of the emission profile of the blazar
Markarian 421 yielding upper limit on the halo flux of the order of 1.5% Crab units in the energy range 300–1300 GeV
