The first is the energy of the particle that produces the gamma ray. Gamma rays result from the interactions of electrons and protons that have been accelerated to almost the speed of light. Higher-energy particles produce higher-energy gamma rays, so one limitation to the maximum gamma-ray energy is the maximum energy to which particles can be accelerated. To accelerate a particle, it must be confined to the region of space where the acceleration takes place using magnetic fields — in the same way as particles are confined to the radius of the Large Hadron Collider here on Earth. The maximum particle energy therefore depends upon the magnetic field strength and the size of the acceleration region. Pulsars, for example, are relatively small but have intense magnetic fields. Galaxy clusters, on the other hand, are enormous but have weak magnetic fields. When a particle reaches a velocity high enough that it can escape the acceleration region, the acceleration stops, and the particle gains no further energy.
The other limitation is the ability of the gamma ray to survive its journey to Earth. Gamma rays are destroyed when they interact with lower-energy photons. Since the universe is full of low-energy photons from starlight and the cosmic microwave background, gamma rays are limited in the distance they can travel. This limit changes with the energy of the gamma ray — low-energy gamma rays can travel across most of the universe; higher-energy gamma rays can only survive if they are produced inside of our galaxy, or in a near neighbor.
Detecting a high-energy gamma ray is relatively simple, but high-energy gamma rays are extremely rare. A typical astronomical source produces just a few gamma rays per square meter of Earth’s surface every year — so we need a very large detector. When a gamma ray strikes the top of the atmosphere, it initiates a cascade of particles, which in turn produces a flash of blue light.
Gamma-ray observatories, such as the Very Energetic Radiation Imaging Telescope Array System (VERITAS) and the High-Altitude Water Cherenkov Gamma-Ray Observatory (HAWC), can observe over effective areas as large as a football field by detecting the products of these particle cascades at ground level. The highest-energy gamma rays that have been detected by these observatories have energies of around 50 teraelectron volts, or 50 billion times the energy of an X-ray. Higher-energy gamma rays certainly exist in the universe, since particles with much higher energies have been observed. Large area cosmic ray observatories, like the Pierre Auger Observatory, are searching for them.
Associate Professor of Physics & Astronomy, University of Delaware, Newark, Delaware