Cosmic Ray Interactions and Gamma-Ray Emission

Cosmic rays are high-energy charged particles, predominantly protons and atomic nuclei, that permeate the Galaxy. As they traverse the interstellar medium, they encounter gas, dust and photon fields, leading to collisions and radiative processes that produce gamma rays. Hadronic interactions between cosmic-ray nuclei and ambient matter generate neutral pions, which decay into gamma-ray photons. Leptonic processes, in particular inverse Compton scattering of low-energy photons by relativistic electrons, also contribute to the high-energy emission. Observations across the MeV to TeV bands—chiefly through space-borne telescopes and ground-based Cherenkov arrays—have mapped the distribution of cosmic rays, revealed their sources and probed their transport properties. Studies now address the balance between particle acceleration at shock fronts, diffusive propagation through magnetised turbulence and energy losses, offering insight into the origin of Galactic and extragalactic cosmic rays. This interplay of cosmic-ray physics and gamma-ray astronomy underpins a broader multimessenger framework, connecting photons, neutrinos and charged particles to elucidate energetic processes in supernova remnants, star clusters, active galactic nuclei and the Galactic centre.

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Cosmic Ray Interactions and Gamma-Ray Emission publication trend

The graph below shows the total number of articles in cosmic ray interactions and gamma-ray emission across all publications each year (not limited to Nature Index journals).

Technical terms

Cosmic ray: A high-energy charged particle, such as a proton or atomic nucleus, originating from astrophysical sources.

Gamma ray: A photon with energy typically above 100 keV, often produced by non-thermal processes in space.

Hadronic interaction: Collision between a cosmic-ray hadron and ambient matter leading to the production of secondary particles, including pions.

Leptonic process: Mechanism in which relativistic electrons scatter low-energy photons (inverse Compton) or emit synchrotron radiation.

Diffusive shock acceleration: A process by which charged particles gain energy through repeated crossings of a shock front in a magnetised plasma.

Interstellar medium (ISM): The diffuse gas, dust and radiation that fill the space between stars in a galaxy.

References

1. Disentangling the Hadronic Components in NGC 1068. The Astrophysical Journal Letters (2023).
2. Understanding the TeV γ-ray emission surrounding the young massive star cluster Westerlund 1. Astronomy & Astrophysics (2023).
3. DEVELOPMENT OF THE MODEL OF GALACTIC INTERSTELLAR EMISSION FOR STANDARD POINT-SOURCE ANALYSIS OF FERMI LARGE AREA TELESCOPE DATA. The Astrophysical Journal Supplement Series (2016).