The Extragalactic Gamma-ray Sky in the Fermi era

TL;DR

This review summarizes seven years of Fermi telescope observations of extragalactic gamma-ray sources, revealing their diversity, aiding in modeling energetic processes, and constraining cosmological models like dark matter.

Contribution

It provides a comprehensive overview of extragalactic gamma-ray sources observed by Fermi, highlighting their spectral diversity, temporal behavior, and implications for cosmology and particle physics.

Findings

Diverse spectral and temporal behaviors of extragalactic gamma-ray sources.

Constraints on dark matter models from gamma-ray population studies.

Indirect measurements of extragalactic background light and intergalactic magnetic fields.

Abstract

The Universe is largely transparent to $\gamma$ rays in the GeV energy range, making these high-energy photons valuable for exploring energetic processes in the cosmos. After seven years of operation, the Fermi {\it Gamma-ray Space Telescope} has produced a wealth of information about the high-energy sky. This review focuses on extragalactic $\gamma$-ray sources: what has been learned about the sources themselves and about how they can be used as cosmological probes. Active galactic nuclei (blazars, radio galaxies, Seyfert galaxies) and star-forming galaxies populate the extragalactic high-energy sky. Fermi observations have demonstrated that these powerful non-thermal sources display substantial diversity in energy spectra and temporal behavior. Coupled with contemporaneous multifrequency observations, the Fermi results are enabling detailed, time-dependent modeling of the energetic particle acceleration and interaction processes that produce the $\gamma$ rays, as well as providing indirect measurements of the extragalactic background light and intergalactic magnetic fields. Population studies of the $\gamma$-ray source classes compared to the extragalactic $\gamma$-ray background place constraints on some models of dark matter. Ongoing searches for the nature of the large number of $\gamma$-ray sources without obvious counterparts at other wavelengths remains an important challenge.