Constraints on the Intergalactic Magnetic Field from Gamma-Ray
  Observations of Blazars
  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.
