At high energies (HE, ~100 MeV–100 GeV), gamma rays can be detected directly by satellites like Fermi-LAT. At very-high energies (VHE, >100 GeV), however, gamma rays from cosmic sources become extremely rare. Building space-based detectors large enough to collect enough signal is impractical, requiring vast, costly collecting areas.

Ground-based observatories overcome this using Imaging Atmospheric Cherenkov Telescopes (IACTs), which detect gamma rays indirectly. When a VHE gamma ray enters Earth’s atmosphere, it creates an extensive air shower—a cascade of secondary particles. Travelling faster than light does in air, these particles emit a faint, nanosecond-long flash of Cherenkov radiation. This blue light spreads over ~250 meters and is too fast and faint for the human eye but detectable by fast, sensitive IACT cameras. By imaging this light, telescopes can reconstruct the gamma ray’s origin and energy.

Current IACT arrays include MAGIC (La Palma, Spain), H.E.S.S., and VERITAS. These instruments have matured the field, enabling the detection of over 250 gamma-ray sources across more than ten classes.  These sources are an important component of the Universe, influencing the evolution of stars and galaxies. At the same time, they also act as a probe of physics in the most extreme environments known – such as in supernova explosions, and around or after the merging of black holes and neutron stars. Despite this progress, major open questions remain: the origin of Galactic cosmic rays at the highest energies, the nature of dark matter, and the mechanisms behind extreme particle acceleration. Addressing these challenges requires a next-generation observatory.

Now is the ideal time to engage with gamma-ray astronomy. The Cherenkov Telescope Array Observatory (CTAO) is under construction and will offer a tenfold improvement in sensitivity, a broader energy range (~20 GeV–300+ TeV), and unprecedented survey capabilities.

Classes of Gamma-Ray Sources:

• Galactic: Pulsars, Pulsar Wind Nebulae, Supernova Remnants, Gamma-ray Binaries, Microquasars, Novae

• Extragalactic: AGN (especially blazars), Starburst Galaxies, GRBs

Other topics at the intersection of gamma-ray astronomy and fundamental physics include searches for dark matter, axion-like particles, and tests of Lorentz Invariance Violation (LIV). In the realm of cosmology, VHE gamma-ray astronomy can contribute to measurements of the Extragalactic Background Light (EBL) and estimations of the Intergalactic Magnetic Field (IGMF).