Diffuse $\gamma$-ray emission from unresolved BL Lac objects

Understand

Blazars represent the most abundant class of high-energy extragalactic $\gamma$-ray sources.

• The subset of blazars known as BL Lac objects is on average closer to Earth and characterized by harder spectra at high energy than the whole sample.
• The fraction of BL Lacs that is too dim to be detected and resolved by current $\gamma$-ray telescopes is therefore expected to contribute to the high-energy isotropic diffuse $\gamma$-ray background (IGRB).
• The IGRB has been recently measured over a wide energy range by the Large Area Telescope (LAT) on board the Gamma-ray Space Telescope ({\it Fermi}).