Extragalactic magnetic fields constraints from simultaneous GeV–TeV observations of blazars

Context. Attenuation of the TeV γ-ray flux from distant blazars through pair production with extragalactic background light leads to the development of electromagnetic cascades and subsequent, lower energy, GeV secondary γ-ray emission. Due to the deflection of VHE cascade electrons by extragalactic magnetic fields (EGMF), the spectral shape of this arriving cascade γ-ray emission is dependent on the strength of the EGMF. Thus, the spectral shape of the GeV–TeV emission from blazars has the potential to probe the EGMF strength along the line of sight to the object. Constraints on the EGMF previously derived from the gamma-ray data suffer from an uncertainty related to the non-simultaneity of GeV and TeV band observations.