PARTICLES AND FIELDS A Tale of Cosmic Rays Narrated in γ Rays by Fermi

Abstract Because cosmic rays are charged particles scrambled by magnetic fields, combining direct measure-ments with other observations is crucial to understanding their origin and propagation. As energetic particles tra-verse matter and electromagnetic fields, they leave marks in the form of neutral interaction products. Among those, γ rays trace interactions of nuclei that inelastically col-lide with interstellar gas, as well as of leptons that undergo Bremsstrahlung and inverse-Compton scattering. Data col-lected by the Fermi large area telescope (LAT) are therefore telling us the story of cosmic rays along their journey from sources through their home galaxies. Supernova rem-nants emerge as a notable γ-ray source population, and older remnants interacting with interstellar matter finally show strong evidence of the presence of accelerated nuclei. Yet the maximum energy attained by shock accelerators is poorly constrained by observations. Cygnus X, a mas-sive star-forming region established by the LAT as housing cosmic-ray sources, provides a test case to study the impact of wind-driven turbulence on the early propagation. Inter-stellar emission resulting from the large-scale propagation of cosmic rays in the Milky Way is revealed in unprece-dented detail that challenges some of the simple assump-tions used for the modeling. Moreover, the cosmic-ray induced γ-ray luminosities of galaxies-scale quasi-linearly with their massive-star formation rates: the overall nor-malization of that relation below the calorimetric limit suggests that for most systems, a substantial fraction of energy in cosmic rays escapes into the intergalactic medium. The nuclear production models and the distribution o