image: The LST-1 telescope, located at the CTAO’s northern array site in La Palma (Canary Islands, Spain), began observing GRB 221009A just 1.33 days after the initial explosion.
Credit: Tomohiro Inada
La Palma, Spain — 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.
The CTAO LST Collaboration is an In-Kind Contributor (IKC) for the CTAO, in charge of building the Large-Sized Telescopes (LSTs). The collaboration is made up of more than 400 scientists and engineers from 67 different institutes across 11 countries: Brazil, Bulgaria, Croatia, Czech Republic, France, Germany, Italy, Japan, Poland, Spain and Switzerland.
About the LST
The Large-Sized Telescopes (LSTs) are one of the three types of telescopes that the CTAO will use to cover its broad energy range, from 20 GeV to 300 TeV. When gamma rays interact with Earth’s atmosphere, they generate cascades of particles that produce Cherenkov light. Because lower-energy gamma rays create only small amounts of Cherenkov light, telescopes with large collection areas are needed to detect it. The LST, with its 23-meter diameter dish, will provide the CTAO’s unique sensitivity in the low-energy range between 20 and 150 GeV.
Despite standing 45 meters tall and weighing 100 tonnes, each LST can reposition to any point in the sky within 20 seconds. Both this rapid repositioning and the low-energy threshold of the LSTs are critical for the CTAO’s studies of galactic transients, high-redshift active galactic nuclei, and gamma-ray bursts.
The CTAO LST Collaboration, responsible for designing and building these telescopes, is making rapid progress on the CTAO-North site in La Palma, Spain. In 2018, the LST prototype, LST-1, was inaugurated and has been under commissioning since then. Currently, three additional LSTs are under construction and are expected to be complete by spring 2026.
About the CTAO
The CTAO (Cherenkov Telescope Array Observatory; www.ctao.org) will be the world’s largest and most powerful observatory for gamma-ray astronomy. The CTAO’s unparalleled accuracy and broad energy range (20 GeV- 300 TeV) will help to address some of the most perplexing questions in astrophysics, falling under three major themes: understanding the origin and role of relativistic cosmic particles; probing extreme environments, such as black holes or neutron stars; and exploring frontiers in physics, searching for dark matter or deviations from Einstein’s theory of relativity. Additionally, the CTAO will play a key role in both multi-wavelength and multi-messenger fields in the coming decades thanks to its enhanced performance, which will allow it to provide fundamental gamma-ray information in the quest to probe the most extreme scenarios.
To cover its broad energy range, the CTAO will use three types of telescopes: the Large-Sized Telescopes (LST), the Medium-Sized Telescopes (MST) and the Small-Sized Telescopes (SST). More than 60 telescopes will be distributed between two telescope array sites: CTAO-North in the northern hemisphere at the Instituto de Astrofísica de Canarias’ (IAC’s) Roque de los Muchachos Observatory on La Palma (Spain), and CTAO-South in the southern hemisphere at the European Southern Observatory’s (ESO’s) Paranal Observatory in the Atacama Desert (Chile). The Headquarters of the CTAO is hosted by the Istituto Nazionale di Astrofisica (INAF) in Bologna (Italy), and the Data Management Centre (SDMC) Data Management Centre (SDMC) is hosted by the Deutsches Elektronen-Synchrotron DESY in Zeuthen (Germany).
The CTAO is a Big Data project. The Observatory will generate hundreds of petabytes (PB) of data in a year (~12 PB after compression). Based on its commitment to open science, the CTAO will be the first gamma-ray observatory of its kind to operate as an open, proposal-driven observatory providing public access to its high-level science data and software products.
In January 2025, the CTAO was established as a European Research Infrastructure Consortium (ERIC) by the European Commission. The Founding Members of the CTAO ERIC are Austria, the Czech Republic, the European Southern Observatory (ESO), France, Germany, Italy, Poland, Slovenia, and Spain. Additionally, Japan as a Strategic Partner, and the accession of Switzerland and Croatia as Founding Members is being processed.
The CTAO ERIC, commonly referred to as the CTAO Central Organisation, is in charge of the construction and operations of the Observatory. This group works in close cooperation with partners from around the world toward the development of the Observatory. Major partners include In-Kind Contribution Collaborations that are developing essential hardware and software, in addition to the CTAO Consortium, an international group of researchers who works in the scientific exploitation of the Observatory.
Contact
Prof. Masahiro Teshima
LST Principal Investigator (PI)
(English, Japanese)
LST Outreach Team
lst-outreach@cta-observatory.org
(English, Italian, Spanish, and Croatian)
Dr. Alba Fernández-Barral
CTAO Chief Communications Officer
alba.fernandezbarral@cta-observatory.org
+39-051-6357-270 (English, Spanish and Italian)
Journal
The Astrophysical Journal Letters
Method of Research
Observational study
Subject of Research
Not applicable
Article Title
GRB 221009A: Observations with LST-1 of CTAO and Implications for Structured Jets in Long Gamma-Ray Bursts
Article Publication Date
23-Jul-2025