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 Roque de los Muchachos observatory (RMO) the most sensitive astronomical instruments in the very-high-energy (VHE) gamma-ray band. MAGIC, which reached in 2024 its 21st year of operation, has been a major contributor to the rapid progress of VHE astronomy in the past two decades – from 6 known sources in 2000, to more than 300 today. IFAE played a central role in the design and construction of MAGIC, as well as in its scientific exploitation, and is now co-leading the construction of the LSTs, the largest among the telescopes of the Northern observatory of CTAO. The first telescope, LST1, is in operation since 2020, and has already obtained its first scientific results, among them the detection of the farthest known VHE source. Three additional LSTs are being built at ORM. CTAO will also have an observatory in the Southern hemisphere (Chile) to achieve full-sky coverage, and will outperform current-generation instruments like MAGIC by an order of magnitude in sensitivity.

Figure 1: The CTAO-LST1 telescope (left, in parking position) and the MAGIC telescopes (right) during an observation. The reddish glow in the background is Aurora Borealis, which reached unusually low latitudes following the exceptional solar storm in May 2024. Credit: Antonio González (Cielos-LaPalma.es).

Activities in 2024

Using ~100 hours of observations collected during its commissioning phase, CTAO-LST1 performed a detailed study of the gamma-ray emission from the Crab pulsar, confirming the power-law spectral tail, and showing the excellent performance of the telescope near its threshold. With CTAO in its construction phase, MAGIC is still producing a wealth of significant results. Among those published in 2024 there are for example new constraints on Lorentz invariance violation obtained from archival data of the active galaxy Mrk 421, as well as an indirect search for signatures induced by axion-like particles in observations of the Perseus galaxy cluster.

Figure 2: Phaseogram of the periodic gamma-ray signal from the Crab pulsar detected by CTAO-LST1 (E>20 GeV)

2024 has been a pivotal year for the construction of the three remaining LSTs at the CTAO northern site. The year began with primarily civil work completed and ended with the three structures assembled on site—one of which already has all its mirrors mounted. The mirrors for the other two telescopes, as well as the cameras, are ready to be installed. IFAE is currently coordinating these efforts. This significant progress, together with the establishment of CTAO as an ERIC, lays the groundwork for exploiting the next generation of Cherenkov telescopes. Additionally, IFAE is contributing to the development of a Raman LIDAR—an auxiliary instrument to be deployed at the CTA-North site to monitor atmospheric conditions during observations.

MAGIC public data legacy

Among the novelties of the CTAO, there will be its operation as an observatory open to the astronomical community, a requirement that prompted gamma-ray astronomers to develop standardised data formats and open-source analysis tools. This new open pipeline constitutes an excellent instrument to disseminate the two decades of observations already gathered by current-generation gamma-ray instruments, so far of proprietary access. IFAE and PIC are leading the implementation of the MAGIC “Data Legacy”, a public archive of all MAGIC stereoscopic observations. The first milestone of this project was achieved this year with the validation of the analysis of 160 hours of standardised MAGIC observations with the aforementioned open pipeline, demonstrating the readiness of the software tools necessary to realise the legacy. To advertise the legacy effort and attract the interest of the community, 60 h of MAGIC data were also, for the first time, released to the public for scientific exploitation.

Thanks to its expertise in software development and data analysis and management, the group joined the EU-funded Astrophysical Centre for Multimessenger studies in Europe (ACME) initiative, within the scope of which it will keep developing the tools for the MAGIC data legacy, organise training events on gamma-ray data analysis, and facilitate virtual and trans-national access to the MAGIC data.

Construction of three additional LSTs at full blast

Figure 4: The four LSTs of the CTAo North observatory by the end of 2024.

During 2024, the LST collaboration has accomplished a big step forward towards having the four LSTs of the CTAO northern site at the RMO operational. The construction has moved from civil works to the actual construction of the structures and the installation of subsystems. The year finished with the three structures ready, two of them with the support for the cameras mounted and one of them with all the mirrors mounted. The rest of the mirrors and the cameras are ready to be installed.