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The international CTAO-LST collaboration provides new data on "GRB 221009A", the brightest gamma-ray burst ever recorded. The results were published on July 23, 2025, in the journal "The Astrophysical Journal Letters" (ApJ Letters).
The publication presents detailed observations conducted in 2022 with the prototype of the Large-Sized Telescope (LST), the LST-1, during its commissioning phase at the Roque de los Muchachos Observatory at the CTAO-North site on the Spanish island of La Palma.
The observations revealed an excess in the gamma-ray flux, which helps to better understand the enigmatic and complex nature of very-high-energy gamma-ray bursts. The results confirm theoretical models suggesting that these bursts generate structured, multi-layered jets in which particles are accelerated.
An extremely bright long gamma-ray burst
Gamma-ray bursts are among the most powerful phenomena in the Universe, releasing in a few seconds an amount of energy equivalent to what the Sun emits over its entire lifetime. As their name suggests, they shine intensely during a brief and rapid phase, lasting from a few seconds to a few minutes, and are followed by an afterglow that can fade over hours, or even months.
Gamma-ray bursts are classified as short or long based on the duration of the flash: long gamma-ray bursts are thought to be linked to exceptionally bright supernovae, while short gamma-ray bursts would result from neutron star collisions. Despite their intense luminosity, these extragalactic sources are difficult 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 October 9, 2022, space observatories, such as NASA's Fermi and Swift satellites, detected an extremely bright long gamma-ray burst, named GRB 221009A. Nicknamed "BOAT" ("Brightest Of All Time"), the burst was so intense that it saturated the multiple instruments observing it and triggered follow-up observations worldwide.
Very stringent upper limits on the very-high-energy gamma-ray flux
The LST-1 telescope began observing the event only 1.33 days after the initial explosion. Spanning more than 20 days after the gamma-ray burst's appearance, the LST-1 observations allowed the LST Collaboration to identify an excess of gamma rays.
Although this excess did not reach the required threshold in the field to claim a formal detection, it allowed the team to establish very stringent upper limits on the very-high-energy gamma-ray flux emitted by the source. These results therefore mark an important step in disentangling competing theoretical models.
Gamma-ray bursts are thought to involve ultra-fast plasma jets ejected either from a black hole or from the merger of neutron stars. However, the exact process of jet formation remains a great mystery. The LST-1 data confirm the theory that gamma-ray burst 221009A was powered by a structured jet: a narrow, ultra-fast core surrounded by a wider, slower sheath of matter.
The validity of this theory, which had been used previously to explain emission from neutron star coalescence, helps to refute the simpler "top-hat" jet theory, commonly used in previous studies, and offers new perspectives on jet formation mechanisms and the nature of the central engine.