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.
