Attenuation of the TeV gamma-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 gamma-ray
emission. Due to the deflection of VHE cascade electrons by extragalactic
magnetic fields (EGMF), the spectral shape of this arriving cascade gamma-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. We investigate constraints on the EGMF
derived from observations of blazars for which TeV observations simultaneous
with those by the Fermi telescope were reported. We study the dependence of the
EGMF bound on the hidden assumptions it rests upon. We select blazar objects
for which simultaneous Fermi/LAT GeV and Veritas, MAGIC or HESS TeV emission
have been published. We model the development of electromagnetic cascades along
the gamma-ray beams from these sources using Monte Carlo simulations, including
the calculation of the temporal delay incurred by cascade photons, relative to
the light propagation time of direct gamma-rays from the source. Constraints on
EGMF could be derived from the simultaneous GeV-TeV data on the blazars RGB
J0710+591, 1ES 0229+200, and 1ES 1218+304. The measured source flux level in
the GeV band is lower than the expected cascade component calculated under the
assumption of zero EGMF. Assuming that the reason for the suppression of the
cascade component is the extended nature of the cascade emission, we find that
B>10^{-15} G (assuming EGMF correlation length of ~1 Mpc) is consistent with
the data. Alternatively, the assumption that the suppression of the cascade
emission is caused by the time delay of the cascade photons the data are
consistent with B>10^{-17} G for the same correlation length.Comment: 9 pages, 9 figure
