Record-breaking gamma-ray burst reveals new clues into cosmic jets
An international team of scientists has announced groundbreaking observations of GRB 221009A, the brightest gamma-ray burst (GRB) ever recorded. Published today in The Astrophysical Journal Letters, these observations provide the strongest evidence yet for the existence of complex, structured jets in long-lived cosmic bursts, considered among the most powerful events in the Universe.
An international team of scientists has announced groundbreaking observations of GRB 221009A, the brightest gamma-ray burst (GRB) ever recorded. Published today in The Astrophysical Journal Letters, these observations provide the strongest evidence yet for the existence of complex, structured jets in long-lived cosmic bursts, considered among the most powerful events in the Universe.
The study, led by the expert Arnau Aguasca-Cabot, from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and the Institute of Space Studies of Catalonia (ICCUB-IEEC), includes the participation of researchers Pol Bordas, Marc Ribó and Josep Maria Paredes (ICCUB-IEEC), among others.
The authors are part of the CTAO LST Collaboration, a global scientific project dedicated to advancing very-high-energy gamma-ray astronomy. The project brings together experts from more than 11 countries to design, build and operate the large-sized telescopes (LSTs) of the Cherenkov Telescope Array Observatory (CTAO), a next-generation facility to explore the most extreme phenomena in the Universe.
GRB 221009A, known as BOAT (for Brightest Of All Time), was first detected on 9 October 2022 by space observatories such as NASA’s Fermi and Swift satellites. The explosion was so intense that it saturated detectors and triggered follow-up observations around the world.
The LST-1 telescope, located in the northern part of the CTAO on the island of La Palma, started observing the event only 1.33 days after the initial explosion. This makes these ground-based observations the earliest very-high-energy gamma-ray observations of this event captured with an atmospheric Cherenkov imaging telescope.
These instruments detect gamma rays indirectly and capture the brief flashes of light that occur when these rays interact with the Earth’s atmosphere. Despite the difficult conditions due to the moonlight, the team was able to record an excess of gamma-ray events from GRB 221009A, making it a rare and valuable discovery in this energy range.
A new window into the physics of cosmic jets
What makes this discovery especially exciting is its contribution to the understanding of GRBs, how they work and how they emit such colossal amounts of energy. The LST-1 data support the theory that GRB 221009A was powered by a structured jet, a narrow, ultrafast core surrounded by a slower, broader sheath. This contrasts with the simpler top-hat jet models that are commonly used to describe GRBs.
The LST-1 observations also help to distinguish between competing theoretical models. Some predicted very high-energy gamma-ray emission, far exceeding what was observed. The new data rule out these models, narrowing the field of study and guiding future studies.
“Unique events like this GRB, which defy theoretical models, may reveal clues about the unknown nature of the central engine that drives this cosmic jet”, says Pol Bordas, ICCUB-IEEC researcher and co-author of the study.
A milestone for the CTAO and high-energy astrophysics
This campaign marks the longest GRB follow-up ever performed by LST-1, lasting more than 20 days. It also demonstrates the telescope’s ability to operate in moonlight conditions, an important step towards increasing the observatory’s responsiveness to transient cosmic events.
“We carried out the first analysis under moonlight conditions, thus establishing a key precedent for the rapid monitoring of transient events when real-time data collection is critical”, says Mònica Seglar, researcher at the Institute of High Energy Physics (IFAE) and coordinator of the study.
These results highlight the power of the CTAO’s new-generation telescopes to explore the high-energy Universe, as research into the inner workings of cosmic explosions is entering a new era of research in unprecedented detail.
As the CTAO continues to expand — with more telescopes operating in both hemispheres — scientists anticipate even faster and more sensitive observations of GRBs and other extreme phenomena.
About the LST
The Large-Sized Telescope (LSTs) are one of the three types of telescopes 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 Canary Islands Institute of Astrophysics’ Roque de los Muchachos Observatory on La Palma (Spain), and CTAO-South in the Southern Hemisphere at the European Southern Observatory’s Paranal Observatory in the Atacama Desert (Chile).
The CTAO Headquarters is hosted by the Istituto Nazionale di Astrofisica (INAF) in Bologna (Italy), and the 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 is 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 Organization, is responsible for 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.
Reference article:
Abe, K., et al. «GRB 221009A: Observations with LST-1 of CTAO and implications for structured jets in long gamma-ray bursts». The Astrophysical Journal Letters, July 2025. DOI: 10.3847/2041-8213/ade4cf.
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