Detections of Fermi-LAT Blazars at the γ-ray Horizon by the EBL Absorption

Extragalactic Background Light (EBL) is the cosmic diffuse electromagnetic radiation that ranges from the far-infrared to the ultraviolet bands, and contains important information on the evolution of the universe. Since during their traveling through the cosmos, the \gamma -ray photons are suffered by the \gamma\gamma absorption by the EBL, investigations of the \gamma -ray opacity of the universe are helpful to understand the EBL. Detections of \gamma rays with significant EBL absorptions (i.e., \tau_\gamma\gamma\gtrsim 1) from distant \gamma -ray sources, is an effective and direct approach to constrain the EBL models. In this study, after careful selections and thorough redshift scrutinization on the sources listed in the latest Fermi-LAT \gamma -ray catalog, we perform systematic Fermi-LAT data analyses on 448 \gamma -ray sources. By comparison with the widely adopted EBL model, 46 horizon \gamma -ray photons from 36 blazars are detected, of which the most distant one is at redshift of 2.944. Among the blazars, the numbers of flat spectral radio quasars (FSRQs) and the BL Lacertae objects are equal. Further temporal \gamma -ray analyses reveal that occasionally arrival of the horizon photons coincide with the unprecedentedly intense \gamma -ray flares. Detections of handful of horizon \gamma -ray photons from the FSRQs put a tight constraint on the location of the energy dissipation region of the jet. Meanwhile, the blazars that emit horizon photons are crucial targets for the observations of the next generation of the space and ground \gamma -ray telescopes.