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

¹ Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics, Nikolaus-Fiebiger-Str. 2, 91058 Erlangen, Germany
² Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany

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Received: 17 October 2025
Accepted: 23 December 2025

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 (Type M, Type B, and Type A), 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), Mono and Stereo. The proposed analysis provides optimal sensitivity over the whole energy range, in contrast to Mono and Stereo, 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.