What question did this study set out to answer?

This analysis aims to integrate different telescope types and trigger strategies in the H.E.S.S. experiment to optimize gamma ray detection across a broad energy range.

January 25, 2026

Event types in H.E.S.S.: A combined analysis for different telescope types and energy ranges

Key Points

This analysis aims to integrate different telescope types and trigger strategies in the H.E.S.S. experiment to optimize gamma ray detection across a broad energy range.
Utilized a large telescope (CT5) in monoscopic mode for low energy detection (≥ 30 GeV).
Employed four smaller telescopes (CT1 to CT4) in stereoscopic mode for higher energy ranges (150 GeV to 100 TeV).
Defined event types based on Hillas parameters to enhance gamma-hadron separation and energy reconstruction.
Compared new analysis configurations to standard configurations in the H.E.S.S. Analysis Package.
Achieved improvements in sensitivity of 25-45% over most energy ranges.
Validated the new analysis using real data from the Crab nebula.
Demonstrated optimal sensitivity across the entire energy range by effectively combining different telescope types.
Observed a wider energy coverage and lower energy threshold with this combined approach.

Abstract

Imaging atmospheric Cherenkov telescopes (IACTs) are the main technique for detecting gamma rays with energies between tens of GeV and hundreds of TeV. One of them, the High Energy Stereoscopic System (H.E.S.S.), has pioneered the use of different telescope types to achieve the broadest possible energy range. A large telesecope with a diameter of 28 , m (CT5) is used in monoscopic mode to access the lowest energies (E ≳ 30 , GeV ), while the four smaller telesceopes with diameters of 12 , m (CT1 to CT4) are used in stereoscopic mode to study energies between 150 GeV and 100 TeV. Nevertheless, a combination of the two telescope types and trigger strategies has proven to be challenging. We propose an analysis based on event types capable of exploiting both telescope types, trigger strategies, and the whole energy range of the experiment for the first time. Because monoscopic and stereoscopic reconstructions are very different, the types are defined based on the Hillas parameters of individual events, resulting in three types ( , , and ), each of which dominates a different energy range. Further improvements to the gamma-hadron separation and to the energy and angular reconstruction are introduced as well. The performances of the new analysis configurations are compared to the standard configurations in the H.E.S.S. Analysis Package (HAP), and . The proposed analysis provides optimal sensitivity over the whole energy range, in contrast to and , which focus on smaller energy ranges. Improvements in sensitivity of 25-45% are also found for most of the energy range. The analysis is validated using real data from the Crab nebula, showing that the application to data of an IACT analysis can combine significantly different telescope types with significantly different energy ranges. A wider energy coverage, a lower energy threshold, a smaller statistical uncertainty for reconstructed spectral parameters, and a higher robustness are observed. The need for a run-by-run correction for the observation conditions is also highlighted. Type M Type B Type A Mono Stereo Mono Stereo

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