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The international CTAO LST Collaboration has released remarkable findings from observations of GRB 221009A—the brightest gamma-ray burst (GRB) ever recorded.

The results were published in .

The publication presents in-depth observations conducted in 2022 with the ) prototype, the LST-1, during its commissioning phase at the Roque de los Muchachos Observatory on the site in La Palma, Spain. The observations revealed a hint of an excess in the gamma-ray flux, which helps 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, remnant 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.