Gamma rays from extragalactic astrophysical sources
  Presently there are several classes of detected gamma-ray extragalatic sources. They are mostly associated to active galactic nuclei (AGN) and (at soft gamma rays) to gamma-ray bursts (GRB), but not only. Active galactic nuclei consist of accreting supermassive black holes hosted by a galaxy that present in some cases powerful relativistic jet activity. These sources, which have been studied in gamma rays for several decades, are probably the most energetic astrophysical objects, and their appearance depends much on whether their jets point to us. Gamma-ray bursts, thought to be associated to collapsing or merging stellar-mass objects at cosmological distances, are also accreting highly relativistic jet sources that shine strongly at high energies. These are very short-duration events, but they are also the most luminous. Recently, star formation galaxies have turned out to be also gamma-ray emitters. On the other hand, clusters of galaxies have not been detected beyond X-rays yet. These are the largest known structures in the Universe; in their formation through accretion and merging, shocks and turbulence are generated, which may lead to gamma-ray production. In this work, the gamma-ray physics of AGNs is briefly presented, as well as that of starburst galaxies, GRBs and clusters of galaxies. Afterwards, we consider some particular cases of gamma-ray production in non-blazar AGN jets interacting with their medium at different scales. 
V. Bosch-Ramon
astro-ph.HE