How ground-based telescopes are tracing the Universe’s most extreme particles

When a very-high-energy gamma ray reaches Earth, it usually does not make it to the ground. Instead, it starts a cascade of particles high in the atmosphere. Those particles produce a brief flash of blue Cherenkov light, which telescopes on the ground can image. By comparing flashes seen by multiple telescopes, researchers can estimate the incoming gamma ray’s energy and direction.

That technique lets ground-based observatories examine gamma rays at energies difficult for space telescopes to collect in large numbers. Because gamma rays are electrically neutral, their paths are not bent by magnetic fields. Their directions can therefore point back toward energetic cosmic environments. But interpreting the light takes care: the telescopes observe the atmospheric shower, not the original gamma ray directly.

Rubén López-Coto’s paper is a plenary review prepared for the 2026 Heidelberg International Symposium on High-Energy Gamma-Ray Astronomy. It brings together selected results from current Imaging Atmospheric Cherenkov Telescope arrays—H.E.S.S., MAGIC and VERITAS—and early science from LST-1, a prototype for the Cherenkov Telescope Array Observatory. The author organizes the review by topic, including novae, jets, pulsars, the Galactic Centre, distant quasars, gamma-ray bursts and dark-matter searches. It does not report a new, unified observing campaign or describe a systematic review of all results in the field.

Explosions, jets and pulsars

One example is the 2021 eruption of the recurrent nova RS Ophiuchi, a system containing a white dwarf and a red giant. The review describes detections by several observatories, including LST-1 measurements over successive nights. It presents the gamma-ray observations as evidence that protons were accelerated in shocks as the nova’s blast wave expanded into gas around the red giant. The review says a proton-acceleration model can account for the observed emission and its evolution; this is an interpretation of the data, not a direct observation of the protons themselves.

Other examples trace energetic particles in space. H.E.S.S. observations of the microquasar SS 433 resolved gamma-ray emission in regions along its jets, far from the central binary. The review describes an energy-dependent pattern in which the highest-energy emission is closer to the jet interaction regions. Models interpret this as evidence about where electrons are accelerated and how they cool while travelling away from those regions.

A striking case is the Vela pulsar, a rapidly rotating neutron star. The review reports pulsed gamma-ray emission extending to 20 teraelectronvolts (TeV), far above the energies expected from some traditional pictures of emission within a pulsar’s magnetosphere. The result challenges those models and points toward acceleration farther out in the pulsar’s wind. The observations establish the high-energy signal; the precise acceleration process remains a physical interpretation.

A changing picture of the Galactic Centre

Earlier observations of diffuse gamma-ray emission near the Galactic Centre had been interpreted as evidence that protons reached the “PeV” range—around a million billion electronvolts. The review discusses newer measurements that favour a high-energy cutoff in the emission, at about 24 TeV. If that preference holds, it could mean the accelerator has a lower maximum energy than previously inferred, or that the most energetic particles have moved out of the region being observed. These are possible explanations, not a settled choice between them.

The review also covers distant quasars, whose gamma rays are absorbed through interactions with the diffuse light between galaxies. Observations of sources near redshift one can therefore help constrain that background light. It discusses gamma-ray afterglows, including observations of GRB 221009A, and combined searches for dark matter using gamma-ray data from multiple observatories. A search that sets limits does not detect dark matter; it narrows the range of signals that particular models can allow.

What the review can—and cannot—show

The paper’s main contribution is synthesis: it connects results from different instruments and research teams to illustrate what very-high-energy gamma-ray astronomy can investigate. The methods described belong to the underlying studies, which use telescope observations, spectral and spatial analyses, and physical models. The review does not provide one shared dataset or a common statistical analysis across all its examples.

That distinction matters. Some conclusions are described strongly, but readers cannot assess every underlying dataset or uncertainty from a summary alone. The Galactic Centre cutoff, for example, is described as a preference, not a definitive measurement of the accelerator’s history. Likewise, proposed mechanisms for emission depend on models as well as observations.

The supplied paper text identifies the work as symposium proceedings and includes references to many of the studies it summarizes. It is not itself a report of peer-review status, and the extracted text may not preserve every figure or qualification. Its claims should therefore be read as an overview of selected results, not as independent confirmation of each one. The broad message is more modest and useful: atmospheric gamma-ray telescopes offer a way to study extreme cosmic environments, while the meaning of each signal depends on careful modelling and continued observation.