Affiliation: Instituto de Astrofísica e Ciências do Espaço – U. de Lisboa

Contribution: Oral

Title: The Cosmic Gamma-Ray Horizon

Abstract: The cosmic gamma-ray horizon is the furthest distance from which very high energy (VHE) gamma-rays are expected to be detected. It results from the fact that gamma-rays with energies above ~30 GeV can be attenuated by the electromagnetic background light (EBL), producing electron-positron pairs.

Therefore, the universe is not transparent to VHE photons, and the gamma-ray emission from extreme sources is expected to be strongly attenuated with distance. Distant gamma-ray sources become a unique tool to measure the EBL density across cosmic time, and can be used to infer the star formation rate (SFR) as far as the epoch of reionization (EoR).

The dominant population of extragalactic gamma-ray emitters are rare Supermassive Black Holes (SMBH) with relativistic jets pointed towards Earth, known as ‘blazars’. Blazars constitute the main tool to study the Cosmic Gamma-Ray Horizon, and are the focus of this work.

In this presentation, I will discuss results from the VHE Horizon Project, which seeks to unveil blazar signatures above energy of 10 GeV. The gamma-ray data analysis integrates 16 years of observations with the Fermi-LAT mission, aiming to identify sources under severe gamma-ray absorption. We catalog those sources, and present new gamma-ray detections that will later improve the statistics of EBL density and SFR measurements. An extensive literature review provided the most up to date redshift description of sources detected above 10 GeV, representing our state-of-the-art view on the cosmic gamma-ray horizon.

This contribution can be found here (pdf).