Fermi Large Area Telescope Detection of Two Very-High-Energy (E>100 GeV) Gamma-ray Photons from the z = 1.1 Blazar PKS 0426-380

Understand

We report the Fermi Large Area Telescope (LAT) detection of two very-high-energy (VHE, E>100 GeV) gamma-ray photons from the directional vicinity of the distant (redshift, z = 1.1) blazar PKS 0426-380.

• The null hypothesis that both the 134 and 122 GeV photons originate from unrelated sources can be rejected at the 5.5 sigma confidence level.
• We therefore claim that at least one of the two VHE photons is securely associated with the blazar, making PKS 0426-380 the most distant VHE emitter known to date.
• The results are in agreement with the most recent Fermi-LAT constraints on the Extragalactic Background Light (EBL) intensity, which imply a $z \simeq 1$ horizon for $\simeq$ 100 GeV photons.