Free Astronomy Magazine September-October 2025

SEPTEMBER-OCTOBER 2025 I mage of LST-1 and the Milky Way at the Roque de los Muchachos Observatory on La Palma. [Daniel López/IAC] not reach the threshold required in the field to claim a formal detec- tion, it allowed the team to estab- lish 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,” says Ramón García López, principal in- vestigator (PI) of the CTAO group at the IAC. 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 neu- tron stars, in short GRBs. However, the exact process behind jet forma- tion 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 ma- terial. This challenges the simpler “top-hat” jet commonly used in ear- lier 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,” points out Juan Cortina, researcher at the IAC/CIE- MAT and Chair of LST Steering Com- mittee. The full moon in the hours following the burst prevented rapid follow-up by other Cherenkov tele- scopes, 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 ob- serving transient cosmic phenom- ena even during very bright moon nights,” adds Cortina. These results demonstrate the pow- er 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 develop- ment by the LST Collaboration on the same site and construction is be- ginning on the CTAO-South site in Chile—intermediate configuration arrays will soon be operational in both hemispheres. With an un- precedented sensitivity, these sub- sets of telescopes will already en- hance our ability to study GRBs and other extreme phenomena. Com- plementarily, the successful deploy- ment of alert handlers is allowing automatic responses, further reduc- ing the follow-up reaction times for transient events. !