Gamma-ray astronomy with ground-based facilities: MAGIC and CTAO

Oscar Blanch, Abelardo Moralejo, Cosimo Nigro

IFAE is a founding member of two international collaborations, MAGIC and CTAO (the Cherenkov Telescope Array Observatory), which have built and operate at the Observatorio del Roque de los Muchachos (ORM, on the island of La Palma) the most sensitive astronomical instruments in the very-high-energy (VHE) gamma-ray band. These facilities exploit the production of Cherenkov light in the atmosphere by the showers of ultra-relativistic particles initiated by VHE photons. MAGIC has been a major contributor to the rapid progress of VHE astronomy in the past 25 years – from barely 6 confirmed sources at the beginning of the century, to more than 300 today. IFAE played a central role in the design, construction and scientific exploitation of MAGIC, and is now co-leading the construction of the Large-Sized Telescopes (LSTs), the largest among the telescopes of the Northern observatory of CTAO. The first telescope, LST-1, is in operation since 2020, and is already producing scientific results. CTAO will also have an observatory in the Southern hemisphere (Chile) to achieve full-sky coverage. Once fully operational, the CTAO telescope arrays will outperform current-generation instruments like MAGIC by an order of magnitude in sensitivity. While projected towards the developments of new instruments, the group focuses part of its effort on the preservation and dissemination of the two decades of data gathered by the MAGIC telescopes.

Activities in 2025

The construction of the three additional LSTs at the ORM advanced significantly through 2025 (see Fig. 1), under the coordination of IFAE, with several major milestones. The mechanical structures were completed, with the installation of the camera support structure of LST-2. All four reflector dishes were also finished, with the installation of the mirror tiles on LST-2 and LST-3. Finally, the next telescope to become operational, LST-4, was also equipped with its camera, making it ready to undergo commissioning. The operation of two or more LSTs in stereoscopic mode will provide a major boost in performance.

During 2025 the MAGIC and LST collaborations continued to perform regular observations with the operational telescopes, including some with a three-telescope system composed of the two MAGICs and LST-1. Several important results from the observations collected in previous years were published. Among others, the detection of the Geminga pulsar by LST-1 (A&A 698), confirming its excellent performance near threshold; the observation of the Nova RS Oph (A&A 695), the first source of its kind in the VHE band; and the follow-up observations of GRB 221009A, the brightest Gamma-Ray Burst ever recorded.

Figure 1: The four Large-Sized Telescopes of the CTAO-North array at the Observatorio del Roque de Los Muchachos as of Summer 2025, with MAGIC-2 visible on the left side. Credit: CTAO-LST collaboration

GRB 221009A, the brightest gamma-ray burst ever recorded. LST-1 provides new clues about Gamma-Ray Burst jets

On October 9th, 2022, space-based observatories, such as NASA’s Fermi and Swift, detected an extremely bright long Gamma-Ray Burst (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. Unfortunately, the event happened on a full moon, during which Cherenkov telescopes do not perform regular observations (because of the high level of scattered moonlight). This prevented the observation on the night immediately following the event, but a dedicated campaign was organized to begin 32 hours after the initial explosion, making LST-1 the first instrument to observe the source in the very high-energy gamma-ray band. Spanning over 20 days after the GRB onset, the observations with LST-1 enabled the LST Collaboration to identify a hint of an excess of gamma rays. While this excess did not reach the threshold required to safely claim a detection, it allowed the team to establish very constraining 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. The results, published on July 23rd, 2025 by The Astrophysical Journal Letters (ApJL 988 L42), support theoretical models in which these bursts generate structured, multi-layered jets where particles are accelerated.

Figure 2: The multi-wavelength light curve of GRB 221009A, “the BOAT”, showing the LST-1 observations together with those of other instruments in the X- and gamma-ray bands, and three proposed models.

1st MAGIC Open Data-Analysis School

Gamma-ray astronomy at the highest energy has been traditionally conducted with proprietary data and software. Before the advent of the next-generation open gamma-ray Observatories like the Cherenkov Telescope Array (CTAO), the MAGIC Collaboration - among others - has devoted significant effort into standardising its own archival observations in order to share them with the Astronomical Community at large. To further and to advertise this long-term objective, in September 2025, the Collaboration under the framework of the ACME project (European Research Executive Agency, grant agreement 101131928) organised its first data-analysis school open to the public. A data set of MAGIC archival observations was publicly released and the participants were guided through several hands-on tutorials representing different gamma-ray analysis scenarios. The tutorials were prepared with the open-source software Gammapy (living in the numpy + astropy open-source ecosystem), the base for the future CTAO science analysis tools. Given the endeavours of the VHE community into standardising data format and analysis tools, the experience acquired in the school will be directly transferable to the analysis of other current or future VHE instruments. We registered a daily attendance of ~50 scientists, mostly early-career, while ~100 people interacted with the school resources offline (lessons, hands-on recording, data, and software remained available after the school).