Gamma rays from distant blazars interact with the extragalactic background light, creating electron-positron pairs, and reducing the gamma-ray flux measured by ground-based atmospheric Cherenkov gamma-ray telescopes. These pairs can Compton-scatter the cosmic microwave background, creating a gamma-ray signature observable by the Fermi Large Area Telesope (LAT).

The signature is also dependent on the intergalactic magnetic field (IGMF), since it can deflect the pairs from our line of sight, reducing the gamma-ray emission. We present preliminary constraints on the IGMF using Fermi-LAT and Cherenkov telescope observations, ruling out both very large and very small values of the IGMF strength.