The extragalactic gamma-ray sky in the Fermi era

  • Francesco MassaroEmail author
  • David J. Thompson
  • Elizabeth C. Ferrara
Review Article


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.


Gamma rays Extragalactic astronomy Active galactic nuclei Quasars BL Lac objects Background light 



Active galactic nucleus(i)


Active galaxy of uncertain type


Blazar candidate of uncertain type


Compton Gamma-Ray Observatory


Compact steep spectrum radio source


Dark matter


Extragalactic gamma-ray background light


Extragalactic gamma-ray background


External Compton


Fanaroff–Riley radio source class


Flat-spectrum radio quasar


Gamma-ray burst


High-frequency peaked BL Lac object


High-synchrotron peaked BL Lac object


Quasi-stellar object (a.k.a. quasar)


Radio galaxy


Radio-loud narrow-line Seyfert of type 1


Spectral energy distribution


Star formation rate


Synchrotron self-Compton


Steep spectrum radio quasar


Unidentified/unassociated gamma-ray source


First Fermi gamma-ray LAT point source catalog


Second Fermi gamma-ray LAT point source catalog


Third Fermi gamma-ray LAT point source catalog


First Fermi LAT AGN catalog


Second Fermi LAT AGN catalog


Third Fermi LAT AGN catalog


Second LAT pulsar catalog


Third EGRET catalog


The combined radio all-sky targeted eight GHz survey


LAT bright AGN sample


Roma-multifrequency blazar catalog


WISE gamma-ray strip


WISE blazar-like radio-loud source catalog


Astrorivelatore Gamma a Immagini Leggero


Australia Telescope Compact Array


Burst Alert Telescope


Cosmic gamma-ray horizon


Celestial Observation Satellite-B


Cherenkov Telescope Array


Energetic Gamma-Ray Experiment Telescope


Fermi all-sky variability analysis


Field of view


GLAST-AGILE support program


Gamma-Ray Burst Monitor


Gamma-Ray Large Area Space Telescope


High-Energy Stereoscopic System


Imager on-Board the INTEGRAL Satellite


INTErnational Gamma-Ray Astrophysics Laboratory


Infrared energy range


Large Area Telescope


Likelihood ratio


Major Atmospheric Gamma-Ray Imaging Cherenkov Telescope


Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments


Orbiting Solar Observatory-3


Point spread function


Small Astronomy Satellite-2


Small and Medium Aperture Research Telescope System


Southern Astrophysical Research Telescope


Ultraviolet energy range


Very Energetic Radiation Imaging Telescope Array System


Very Large Array


Very Long Baseline Array


Very long baseline interferometry


Whole Earth Blazar Telescope



This review would not have been possible without the dedicated efforts of scientists, engineers, and technicians who have made the Fermi Gamma-ray Space Telescope mission so successful. We extend thanks to all those who contributed. Special thanks to Justin Finke, Filippo D’Ammando and Seth Digel for valuable comments on the manuscript. F. Massaro wishes to thank M. Ajello, R. D’Abrusco, D. Gasparrini, M. Giroletti, L. Latronico, N. Masetti, A. Paggi, H. Smith and G. Tosti for their support during the last 4 years spent working on Fermi blazars. The work by is supported by the Programma Giovani Ricercatori—Rita Levi Montalcini—Rientro dei Cervelli (2012). This review is also supported by the NASA grants NNX12AO97G and NNX13AP20G. Part of this work is based on archival data, software or on-line services provided by the ASI Science Data Center. This research has made use of data obtained from the high-energy Astrophysics Science Archive Research Center (HEASARC) provided by NASA’s Goddard Space Flight Center; the SIMBAD database operated at CDS, Strasbourg, France; the NASA/IPAC Extragalactic Database (NED) operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. TOPCAT ( (Taylor 2005) for the preparation and manipulation of the tabular data and the images. The Aladin Java applet ( was used to create the finding charts reported in this paper (Bonnarel 2000). It can be started from the CDS (Strasbourg, France), from the CFA (Harvard, USA), from the ADAC (Tokyo, Japan), from the IUCAA (Pune, India), from the UKADC (Cambridge, UK), or from the CADC (Victoria, Canada).


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Copyright information

© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  • Francesco Massaro
    • 1
    Email author
  • David J. Thompson
    • 2
  • Elizabeth C. Ferrara
    • 2
  1. 1.Dipartimento di FisicaUniversità degli Studi di TorinoTurinItaly
  2. 2.NASA Goddard Space Flight CenterGreenbeltUSA

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