The extragalactic gamma-ray sky in the Fermi era.

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 7 years of operation, the Fermi 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 remain an important challenge. [ABSTRACT FROM AUTHOR]