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

Understand

Distant BL Lacertae objects emit $\gamma$ rays which interact with the extragalactic background light (EBL), creating electron-positron pairs, and reducing the flux measured by ground-based imaging atmospheric Cherenkov telescopes (IACTs) at very-high energies (VHE).

• These pairs can Compton-scatter the cosmic microwave background, creating a $\gamma$-ray signature at slightly lower energies observable by the \fermi\ Large Area Telescope (LAT).
• This signal is strongly dependent on the intergalactic magnetic field (IGMF) strength ($B$) and its coherence length ($L_B$).
• We use IACT spectra taken from the literature for 5 VHE-detected BL Lac objects, and combine it with LAT spectra for these sources to constrain these IGMF parameters.