A tale of cosmic rays narrated in gamma rays by Fermi

Abstract

Because cosmic rays are charged particles scrambled by magnetic fields, combining direct measurements with other observations is crucial to understanding their origin and propagation. As energetic particles traverse matter and electromagnetic fields, they leave marks in the form of neutral interaction products. Among those, gamma rays trace interactions of nuclei that inelastically collide with interstellar gas, as well as of leptons that undergo Bremsstrahlung and inverse-Compton scattering. Data collected 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 remnants emerge as a notable gamma-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. Cygnus X, a massive 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. Interstellar emission resulting from the large-scale propagation of cosmic rays in the Milky Way is revealed in unprecedented detail that challenges some of the simple assumptions used for the modeling. Moreover, the cosmic-ray induced gamma-ray luminosities of galaxies scale quasi-linearly with their massive- star formation rates, and 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 of target gas and radiation fields, not determined precisely enough yet, are key to exploiting the full potential of gamma-ray data. Nevertheless, data being collected by Fermi and complementary observations are bringing us ever closer to solving the cosmic-ray mystery. (abridged)

Write-up from highlight talk at the 33rd ICRC, Rio de Janeiro, July 2013. To appear in Brazilian Journal of Physics. 8 pages, 4 figures

References in corpus (16)

• Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind?
• Detection of the Characteristic Pion-Decay Signature in Supernova Remnants
• Primary particle acceleration above 100 TeV in the shell-type Supernova Remnant RX J1713.7-3946 with deep H.E.S.S. observations
• A connection between star formation activity and cosmic rays in the starburst galaxy M 82
• Global cosmic-ray related luminosity and energy budget of the Milky Way
• Observations of the young supernova remnant RX J1713.7-3946 with the Fermi Large Area Telescope
• Discovery of TeV Gamma-Ray Emission from the Cygnus Region of the Galaxy
• Chemistry of the Sagittarius Dwarf Galaxy: a Top-Light IMF, Outflows and the R-Process
• Search for cosmic-ray induced gamma-ray emission in Galaxy Clusters
• Detection of extended very-high-energy gamma-ray emission towards the young stellar cluster Westerlund 2
• Mechanism for spectral break in cosmic ray proton spectrum from Supernova remnant W44
• Origin of Cosmic Rays
• A common solution to the cosmic ray anisotropy and gradient problems
• Observations of the shell-type SNR Cassiopeia A at TeV energies with VERITAS
• Gamma-ray observations of the Orion Molecular Clouds with the Fermi Large Area Telescope
• On the Physics Connecting Cosmic Rays and Gamma Rays: Towards Determining the Interstellar Cosmic Ray Spectrum

Cited by in corpus (5)

• First Predicted Cosmic Ray Spectra, Primary-to-Secondary Ratios, and Ionization Rates from MHD Galaxy Formation Simulations
• Standard Self-Confinement and Extrinsic Turbulence Models for Cosmic Ray Transport are Fundamentally Incompatible with Observations
• Constraining Cosmic-ray Transport with Observations of the Circumgalactic Medium
• Spectrum of Relativistic and Subrelativistic Cosmic Rays in the 100 pc Central Region
• The radial gradient of cosmic ray intensity in the Galaxy