The Sky Seen in γ-Rays

The presence of galactic magnetic fields makes it impossible to localize Cosmic Ray (CR) sources using charged particles. The only way to gain information about their acceleration sites is by observing the neutral particles (γ-rays and neutrinos) generated by their interactions during acceleration. In recent years, a new window has been opened on the observation of the electromagnetic radiation up to the highest energies. The development has been made possible by the availability of new detectors coming from technologies typical of experimental particle physics. In most cases, electromagnetic radiation processes involving relativistic electrons could explain the photon flux up to the highest energies, which presents a non-thermal emission with a distribution with two distinct features. High-energy photons can be produced as well by accelerated protons though decay of secondary neutral mesons. In addition to physical mechanisms, in this chapter we describe the main experiments that allowed γ-ray astronomy: the Compton Gamma Ray Observatory, the Swift, AGILE and Fermi satellites. Unlike the sky at visible wavelengths, the γ-ray sky is dominated by a diffuse radiation originating in our Galaxy, due to the propagation of CRs in the interstellar medium. In most cases, galactic and extragalactic sources appear as point-like objects over the diffuse γ-ray background. In addition to these steady sources, flashes of gamma-rays were discovered serendipitously as early as the beginning of the 1970s. These Gamma Ray-Bursts (GRBs) are the brightest explosions in the Universe, observed at a rate of about 1/day. Their origin, classification, total energy output and the γ-ray differential flux have been experimentally investigated only recently. These objects are possible candidates as sources of ultra-high energy cosmic rays.