Astronomy and Astrophysics Review ( IF 15.143 ) Pub Date : 2015-12-19 , DOI: 10.1007/s00159-015-0090-6
Francesco Massaro, David J. Thompson, Elizabeth C. Ferrara

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.

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