H.E.S.S. and CTA


This image illustrates all three classes of the CTAO telescopes. While not an accurate representation of the final array layout, this rendering illustrates the enormous scale of the CTAO telescopes and the array, itself. Credit: Gabriel Pérez Diaz, IAC / Marc-André Besel, CTAO

Cosmic accelerators and associated phenomena can be probed using Very-High-Energy (VHE) gamma rays, i.e. within the 100 GeV – 100 TeV energy range. VHE gamma-rays are so rare that their detection requires collection areas of tens of hectares (~10^5 m^2), well beyond what is achievable with satellite-borne instruments (~1m^2).

By taking advantage of the Cherenkov effect the ground-based observatories H.E.S.S and CTA can reach such large collection areas simply, by using the atmosphere as part of the instrument. Gamma rays, and more generally charged cosmic rays (ions or electrons) which impinge on the Earth’s atmosphere interact with Nitrogen and Oxygen nuclei, and deposit their energy through generation of particle cascades — called air showers. The shower particles are so energetic (or ultra-relativistic) that they go faster than light in the atmosphere. This creates an electromagnetic shock-wave along the trajectory of the particles, which manifests as an ultra-brief light emission, a phenomenon known as Cherenkov light. The telescopes are designed to collect this light using ultra-fast shutter speed cameras. The latter usually consist of an array of photomultipliers tubes (PMTs), or silicon photomultipliers (SiPMs). The images of the same shower collected by multiple telescopes are used to reconstruct the initial particle direction and energy, but also to estimate its probability to be a genuine gamma ray in order to reject the background of charged cosmic rays.


The High Energy Stereoscopic System (H.E.S.S.) is an array of 5 imaging Cherenkov telescopes operating since 2002 in the Khomas Highland of Namibia. The H.E.S.S collaboration consists of over 200 scientists from Germany, France, the United Kingdom, Namibia, South Africa, Ireland, Armenia, Poland, Australia, Austria the Netherlands, Japan and Sweden, supported by their respective funding agencies and institutions. The H.E.S.S array consists of four 13 meter diameter telescopes, each equipped with a 960-pixel (photomultiplier tubes or PMTs) fast cameras and surrounding a large telescope with a 28 meter dish and a high resolution 2000 PMT camera. The APC team is one of the major contributing groups to the H.E.S.S. project since its design back in the late 1990’s and is involved in many science topics investigated with this major facility in the VHE domain and through multi-wave length and multi-messenger observations, e.g. the origin of cosmic rays, the acceleration mechanisms, accretion/ejection, radiative and propagation processes at work in diverse astrophysical objects in the the Galaxy such as pulsars, supernova remnants and bubbles, but also in the vicinity of the central black hole of the Galaxy or near super massive black holes and in Jets of Active Galactic Nuclei, etc.

Flux map of the H.E.S.S. Galactic Plane Survey. This observation reveals 78 objects emitting in the TeV range, among which 10 were previously unknown.


The team members: Y. Beccherini, M. Cerruti,, A. Djannati- Ataï, S. Gabici, L. Giunti (Post-doc 2022), B. Khélifi, A. Lemière, M. Regeard (PhD 2025), S. Pita, M. Punch, R. Terrier, S. Zouari (PhD 2022).

Former members: J. Devin (Post-doc, 2021) M. Spir-Jacob (PhD 2019) A. Sinha (Post-doc, 2017-2018)


APC is involved in the Cherenkov Telescope Array (CTA) project. Following on from the current generation of gamma-ray telescopes in operation, such as the H.E.S.S network in Namibia, CTA is an international project led by more than 30 countries aiming to build the largest and most sensitive gamma-ray observatory in the very-high- energy range (VHE, above 30-100 GeV) with dozens of telescopes planned at two sites, one in each hemisphere (Island of La Palma, Spain, and near Paranal, Chile). APC members are actively committed to the preparation of the science with CTA, with major interests to Galactic science, e.g. SNRs, pulsars, propagation and diffuse emission, search for PeVatrons, high-energy phenomena in the vicinity of the Galactic Center) as well as extragalactic subjects such as AGN jets, blazar variability and gamma-ray cosmology. The APC team contributes to the system called UCTS (Unified Clock distribution and Trigger time Stamping) designed to allow the synchronisation of the telescopes in the array and provide accurate timestamps. To implement the UCTS concept, APC has developed the TiCkS (Timing and Clock Stamping) board, based on the White Rabbit system (CERN).

APC is also involved in Gammapy project. As an open source Python package, Gammapy, was designed as a prototype of a future analysis software for CTA, but also with a view to sharing with other research groups in gamma astronomy. Indeed CTA will be mainly operated as a proposal-based open observatory, with public access to the observational data for scientific users, together with openly accessible analysis software. Gammapy has been selected on June 2021 as the core library for the CTA Science Analysis tools. As an open observatory, CTA offers also several services through the centralized web Science Gateway which provides an access with a single identification to all CTA applications. In this context, the APC IT Department is committed to implement the Proposal Handling Platform for scientific observations to the CTA consortium.

The team members: Y. Beccherini, M. Cerruti,, A. Djannati-Ataï, S. Gabici, L. Giunti (Post-doc 2022), B. Khélifi, A. Lemière, A. Neronov, M. Regeard (PhD 2025), S. Pita, M. Punch, D. Semikoz, R. Terrier, S. Zouari (PhD 2022).

Former members: J. Devin (Post-doc, 2021) M. Spir-Jacob (PhD 2019) A. Sinha (Post-doc, 2017-2018) CTA Observatory South site in Chile.

Early group history

The VHE gamma-ray team at APC (formerly at the PCC laboratory at Collège de France) is among the founders of this domain of research starting its activities back in the mid-1980’s with the THEMISTOCLE experiment in the French Pyrénées. This experiment used and recycled the former Thémis solar plant infrastructure. The ASGAT experiment led by CEA was also taking place on the same site. These first investigations resulted into the detection of the Crab Nebula in the TeV range, confirming its first discovery in the VHE range by the pioneering experiment at the Whipple observatory, USA. The team then contributed to the design and operation of the CAT  project (Cherenkov Array at Themis), a 5-metre diameter dish equipped with fast electronics and a high-resolution camera (600 photo-tubes), for a monoscopic imaging Cherenkov telescope with the highest pixel density to date.

APC contributed also to the CELESTE experiment of which the goal was to bridge the gap between satellite-borne gamma-ray detectors operating in the high-energy range (HE; 100 MeV – 30 GeV) and ground-based telescopes.

In late 1990’s the APC team was involved in the preparation of the next generation of experiments through participation to the design and construction of the H.E.S.S. project. H.E.S.S. consists of an array of 5 imaging Cherenkov telescopes (or IACTs) based in the Khomas Highland of Namibia. H.E.S.S. is in operation since 2002 and has produced ground-breaking discoveries in the domain. Since mid 2000’s the APC team has engaged in the CTA project which consists of tens of Cherenkov telescopes and will be the next major and world-level facility for VHE gamma-ray astrophysics.