Abstract
Continuum gamma-ray emission produced by interactions of cosmic rays with interstellar matter and radiation fields is a probe of non-thermal particle populations in galaxies. After decades of continuous improvements in experimental techniques and an ever-increasing sky and energy coverage, gamma-ray observations reveal in unprecedented detail the properties of galactic cosmic rays. A variety of scales and environments are now accessible to us, from the local interstellar medium near the Sun and the vicinity of cosmic-ray accelerators, out to the Milky Way at large and beyond, with a growing number of gamma-ray emitting star-forming galaxies. Gamma-ray observations have been pushing forward our understanding of the life cycle of cosmic rays in galaxies and, combined with advances in related domains, they have been challenging standard assumptions in the field and have spurred new developments in modelling approaches and data analysis methods. We provide a review of the status of the subject and discuss perspectives on future progress.
Highlights
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In the GeV energy range we find the peak of energy output from Cosmic rays (CRs) interactions in the interstellar medium (ISM), interstellar emission prevails over discrete sources, and it is dominated by hadronic emission correlated with interstellar gas
CRs in the vicinity of sources can be associated with specific populations of gamma-ray sources or specific regions of the ISM, and we review below the current knowledge on such objects for three categories: emission beyond the shock in supernova remnants (SNR), emission around pulsars and their nebulae, and extended emission coincident with star-forming regions (SFRs)
Summary
Cosmic rays (CRs) are energetic particles first observed around the Earth with energies ranging from MeV to above 1020 eV and with approximately isotropic arrival directions. Gamma-ray observations at lower energies require the use of space-borne telescopes exploiting different detection techniques: coded masks in the energy range from hundreds of keV to MeV and Compton detectors at MeV energies In this domain the state-of-the-art instruments date back to twenty or even thirty years ago with INTEGRAL SPI [20] (for INTEGRAL legacy results see [21]), and COMPTEL [22]. CTA will feature more than one hundred IACTs of different sizes located on two sites in the northern and southern hemispheres, it will be able to observe the entire sky It will cover the energy range from a few tens of GeV to >300 TeV with a sensitivity one order of magnitude better than current IACTs, a field of view reaching 10◦, reduced CR background, and an angular resolution of a few arcmin. This technique is well suited for the study of large-scale emission
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