High Energy Astrophysics and Galactic Cosmic Rays
Summary
High energy astrophysics studies the most violent processes in the Universe, from supernova explosions and pulsar winds to active galactic nuclei and cosmic‐ray acceleration. Galactic cosmic rays—relativistic protons, nuclei and electrons—pervade the Milky Way, carrying an energy density comparable to that of starlight or magnetic fields. Their spectrum spans over ten orders of magnitude, with pronounced features at a few petaelectronvolts (“knee”) and several exaelectronvolts (“ankle”). Below the knee, cosmic rays are widely believed to be accelerated in the shock waves of supernova remnants and in collective stellar–wind environments; above the ankle, extragalactic sources such as radio galaxies and gamma‐ray bursts take over. While charged cosmic rays undergo diffusion in the tangled Galactic magnetic field—erasing direct source information—high‐energy gamma rays betray the sites of particle acceleration. Leptonic processes (synchrotron emission and inverse Compton scattering) and hadronic interactions (π⁰ decay following proton–proton collisions) produce gamma rays from tens of gigaelectronvolts to beyond a petaelectronvolt. Ground‐based observatories exploit the Cherenkov light of extensive air showers and large water–Cherenkov detectors to map both point‐like and diffuse emission, revealing populations of pulsar wind nebulae, supernova remnants, star‐forming regions and extragalactic jets. These measurements constrain magnetic‐field amplification, diffusion coefficients, maximum energies and the transition from Galactic to extragalactic cosmic rays, while also probing the interstellar and intergalactic photon fields through γγ absorption.
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High Energy Astrophysics and Galactic Cosmic Rays publication trend
The graph below shows the total number of articles in high energy astrophysics and galactic cosmic rays across all publications each year (not limited to Nature Index journals).
Technical terms
PeVatron: An astrophysical accelerator capable of boosting particles to petaelectronvolt energies.
Inverse Compton scattering: The process by which relativistic electrons transfer energy to low-energy photons, elevating them into the X-ray or gamma-ray band.
Diffusion coefficient (D): A parameter describing the rate at which cosmic rays scatter and spread through turbulent magnetic fields; often energy-dependent as D∝Eδ.
π⁰ decay: The rapid decay of neutral pions—produced in proton–proton collisions—into two gamma-ray photons, a key hadronic emission mechanism.
References
- An Introduction to Very-High-Energy Astrophysics.
- Ultrahigh-energy photons up to 1.4 petaelectronvolts from 12 γ-ray Galactic sources. Nature (2021).
- Gamma-ray haloes around pulsars as the key to understanding cosmic-ray transport in the Galaxy. Nature Astronomy (2022).
- Insights from LHAASO and IceCube into the origin of the Galactic diffuse teraelectronvolt–petaelectronvolt emission. Nature Astronomy (2024).
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