Revision of conservative lower bound on the intergalactic magnetic field from Fermi and Cherenkov telescope observations of extreme blazars

Context. Joint observations of extreme blazars with the Fermi Large Area Telescope (LAT) and Imaging Atmospheric Cherenkov telescopes (IACTs) have previously been used to derive lower bounds on the intergalactic magnetic field (IGMF). Aims. We aim to update these previous bounds using a set of extreme blazars that were detected in the very-high-energy (VHE, photon energies above 100 GeV) band by both Fermi/LAT and IACTs. Methods. We measured IGMF-dependent suppression of secondary delayed γ -ray flux from electron-positron pairs deposited in the intergalactic medium by VHE γ -rays interacting with extragalactic background light. Results. From a total of 22 extreme blazars detected by Fermi/LAT and IACTs in the VHE band, seven have spectral characteristics inconsistent with the possibility of zero magnetic field along their lines of sight, even under the most restrictive assumption that the sources only switched on at the start of VHE band observations. Adopting this assumption, we derive a “conservative” lower bound on the IGMF strength at the level of ∼2 × 10 −17 G. The tightest bound is imposed by the signal of 1ES 0502+675, a source that has not been considered in the IGMF analysis before. Our bound is comparable to the bound derived by MAGIC collaboration, but it is weaker than that previously derived from analyses of Fermi/LAT and HESS telescope data, even though our dataset includes those data. We clarify the origin of this discrepancy.