LST Collaboration Paper Provides New Clues About Gamma-Ray Burst Jets
New data on gamma-ray burst ‘GRB 221009A’, observed in October 2022, confirms theoretical models suggesting that these exceptionally intense bursts of electromagnetic waves generate structured, multi-layered jets in which particles are accelerated.
The international CTAO LST Collaboration released remarkable findings from observations of GRB 221009A—the brightest gamma-ray burst (GRB) ever recorded. The results were published on 23 July by the renowned journal The Astrophysical Journal Letters (ApJ Letters). The publication presents in-depth observations conducted in 2022 with the Large-Sized Telescope (LST) prototype, the LST-1, during its commissioning phase at the Roque de los Muchachos Observatory on the CTAO-North site in La Palma, Spain. The observations revealed a hint of an excess in the gamma-ray flux, which help provide new insights into the enigmatic and complex nature of GRBs at very high energies. The results support theoretical models in which these bursts generate structured, multi-layered jets where particles are accelerated.
GRBs are among the Universe’s most powerful phenomena, releasing in just seconds as much energy as the Sun emits over its entire lifetime. As their name suggests, they burst over a brief, prompt phase, lasting seconds to minutes, and then are followed by an afterglow that can fade over hours to months. GRBs are classified as short or long based on the duration of the burst: long GRBs are thought to be linked to exceptionally bright supernovae, while short GRBs likely result from neutron star collisions. Despite their intense brightness, these extragalactic sources are challenging to detect at the highest energies because the gamma rays they emit weaken over the vast distances they travel, as well as due to their transient nature.
On 9 October 2022, space-based observatories, such as NASA’s Fermi and Swift satellites, detected an extremely bright long GRB, named GRB 221009A. Dubbed the “BOAT” (“Brightest Of All Time”), the burst was so intense that it saturated multiple instruments observing it, and triggered follow-up observations across the globe.
The LST-1 telescope, located at the CTAO’s northern array site in La Palma (Canary Islands, Spain), began observing the event just 1.33 days after the initial explosion. Spanning over 20 days after the GRB onset, the observations with the LST-1 enabled the LST Collaboration to identify an excess of gamma rays. While this excess did not reach the threshold required in the field to claim a formal detection, it allowed the team to establish very constrained upper limits on the very high-energy gamma-ray flux emitted by the source. Thus, these results mark an important step toward disentangling between competing theoretical models.
GRBs are believed to involve ultra-fast jets of plasma ejected either from a black hole, remanent of long GRBs, or from the merging of neutron stars, in short GRBs. However, the exact process behind jet formation remains a major mystery. The LST-1 data support the theory that GRB 221009A was powered by a complex, structured jet: a narrow, ultra-fast core surrounded by a wider, slower-moving sheath of material. This challenges the simpler “top-hat” jet commonly used in earlier studies and offers new insights into jet formation mechanisms and the nature of the central engine.
Notably, the recorded data include observations made under very bright moonlight conditions, which poses a significant challenge for Cherenkov telescopes due to their sensitive cameras. The full moon in the hours following the burst prevented rapid follow-up by other Cherenkov telescopes, but the technical solutions developed by the LST Collaboration made it possible for the LST-1 to be the first one to observe the source in the very high-energy gamma-ray regime. This marks the first time that the LST-1 has collected data under such challenging conditions, opening new possibilities for observing transient cosmic phenomena even during very bright moon nights.
These results demonstrate the power of the CTAO’s next-generation telescopes to explore the very high-energy Universe, ushering in a new era where researchers can probe the inner workings of cosmic sources in unprecedented detail. As the CTAO continues to expand—three more LSTs are under development by the LST Collaboration on the same site and construction is beginning on the CTAO-South site in Chile—intermediate configuration arrays will soon be operational in both hemispheres. With an unprecedented sensitivity, these subsets of telescopes will already enhance our ability to study GRBs and other extreme phenomena. Complementarily, the successful deployment of alert handlers is allowing automatic responses, further reducing the follow-up reaction times for transient events.
About CTAO
The Cherenkov Telescope Array Observatory (CTAO; www.cta-observatory.org) will be the first open ground-based gamma-ray observatory and the world’s largest and most sensitive instrument for the exploration of the high-energy Universe. The CTAO’s unparalleled accuracy and broad energy range (20 GeV- 300 TeV) will provide novel insights into the most extreme and powerful events in the Cosmos, addressing questions in and beyond astrophysics falling under three major themes: Understanding the origin and role of relativistic cosmic particles, probing extreme environments (such as black holes and neutron stars) and exploring frontiers in physics (such as the nature of dark matter). To do so, the CTAO has two telescope array sites: CTAO-North in the northern hemisphere at the Instituto de Astrofísica de Canarias’s (IAC’s) Roque de los Muchachos Observatory on La Palma (Spain), and CTAO-South in the southern hemisphere near the European Southern Observatory’s (ESO’s) Paranal Observatory in the Atacama Desert (Chile). The headquarters is hosted by the Istituto Nazionale di Astrofisica (INAF) in Bologna (Italy), and the Science Data Management Centre (SDMC) is hosted by the Deutsches Elektronen-Synchrotron (DESY) in Zeuthen (Germany). The CTAO will also be the first observatory of its kind to be open to the worldwide scientific communities as a resource for data from unique, high-energy astronomical observations.
The Board of Governmental Representatives (BGR) is the committee preparing for the CTAO’s legal status transition, formed by 12 countries and one intergovernmental organisation: Australia, Austria, Brazil, Czech Republic, European Southern Observatory (ESO), France, Germany, Italy, Japan, Poland, Slovenia, Spain and Switzerland. The CTAO gGmbH Council is the gGmbH’s governing body, composed of shareholders from 11 countries and one intergovernmental organisation, as well as associate members from two countries.
The CTAO gGmbH works in close cooperation with partners from around the world toward the development of the Observatory. Major partners include In-Kind Contribution teams, such as the telescope teams that are developing essential hardware and software, in addition to the science collaboration, an international group of researchers who have provided scientific guidance since the project's inception.
The CTAO was promoted to a “Landmark” on the European Forum on Research Infrastructure (ESFRI) Roadmap 2018, and was ranked as the main priority among the new ground-based infrastructures in the ASTRONET Roadmap 2022-2035.
12 CNRS laboratories involved
CNRS - IN2P3 : le laboratoire Astroparticule et cosmologie (APC, Paris), le Centre de physique des particules de Marseille (CPPM, Marseille), le laboratoire de physique des 2 infinis Irène Joliot Curie (IJCLab, Orsay), le Laboratoire d'Annecy de physique des particules (LAPP, Annecy), le Laboratoire Leprince Ringuet (LLR, Palaiseau), le Laboratoire des 2 infinis Bordeaux (LP2I Bordeaux), le Laboratoire de physique nucléaire et des hautes énergies (LPNHE, Paris), le Laboratoire Univers et particules de Montpellier (LUPM, Montpellier).
CNRS - INSU : l'Institut de planétologie et d'astrophysique de Grenoble (IPAG, Grenoble), L'Institut de recherche en astrophysique et planétologie (IRAP, Toulouse), le Laboratoire Univers et Théories (LUTH, Meudon), l'Observatoire de la Côte d'Azur (OCA, Nice).
The French contribution to CTAO
The French scientific and technical staff involved in CTAO come from CNRS laboratories (CNRS Nuclear & Particles and CNRS Earth & Universe) and IRFU at CEA Paris-Saclay. They contribute to many aspects of the infrastructure's implementation and scientific operation. A significant part of the contribution concerns the three types of telescopes, dozens of which will equip the Observatory's two networks, detecting Cherenkov light produced by the interaction of gamma rays in the atmosphere, between 10 GeV and 100 TeV. More specifically, the French contribution involves: for the LSTs, the design of the arches, the camera controllers and the motorisation of the mechanical structure; for the MSTs, the design, construction and integration of fast electronics cameras (NectarCAM) and the design and supply of mirrors; for the SSTs, the design and integration of the telescopes. French teams are also contributing to monitoring atmospheric quality by building a LIDAR on the southern site. Finally, they are playing a leading role in deploying the data analysis chain for the telescope networks and scientific analysis tools for the sky for future users of the Observatory.
About the CTAO-LST collaboration
Large-sized telescopes (LSTs) are one of three types of telescopes that the CTAO observatory will use to cover its wide energy range from 20 GeV to 300 TeV. When gamma rays interact with the Earth's atmosphere, they generate particle cascades that produce Cherenkov light. Since low-energy gamma rays produce only small amounts of Cherenkov light, telescopes with a large collecting area are needed to detect them. The LST, with its 23-metre diameter dish, will provide the CTA observatory with exceptional sensitivity in the low-energy range, between 20 and 150 GeV.
Despite their imposing dimensions (45 metres high and weighing 100 tonnes), each LST can reposition itself to any point in the sky in 20 seconds. This rapid repositioning and the low energy threshold of the LSTs are essential for the CTA observatory's studies of galactic transients, high-redshift active galactic nuclei and gamma-ray bursts.
The CTAO LST collaboration is a contributor in kind to the CTA observatory, responsible for the construction of the LSTs. The collaboration comprises more than 400 scientists and engineers from 67 different institutes in 11 countries: Germany, Brazil, Bulgaria, Croatia, Spain, France, Italy, Japan, Poland, the Czech Republic and Switzerland. CNRS Nuclear & Particles contributes to the collaboration's work through the CPPM and LAPP laboratories.