CTA: Cherenkov Telescope Array

Manel Martínez

The Cherenkov Telescope Array (CTA) is the next generation ground-based observatory for gamma-ray astronomy at very-high energies. With more than 100 telescopes located in the northern (ORM Observatory, La Palma) and southern (ESO Paranal Observatory, Chile) hemispheres, CTA will be the world’s largest and most sensitive high-energy gamma-ray observatory.

Introduction

CTA will study cosmic particle accelerators that can reach energies inaccessible to man-made accelerators like the LHC

CTA will observe the sky at an unprecedented very high energy. In fact, the cosmic particle accelerators CTA will probe can reach energies inaccessible to man-made accelerators like the Large Hadron Collider. CTA’s unique capabilities will help us to address some of the most perplexing questions in astrophysics. CTA will seek to understand the impact of high-energy particles in the evolution of cosmic systems and to gain insight into the most extreme and unusual phenomena in the Universe. CTA will search for annihilating dark matter particles and deviations from Einstein’s theory of relativity and even conduct a census of particle accelerators in the Universe.

Progress of the CTA project in 2018

During 2018 the main progress in the CTA project in what concerns politics has been the signature in December of the site agreement for CTA South with the Chilean Government and ESO and, in what concerns construction, the completion and inauguration in October of the first LST prototype at La Palma (see figs.1 and 2), de-facto the first CTA telescope in a CTA observatory site.

Figure 1: The LST1 by the beginning of October 2018, ready for the inauguration, with a panel showing all the institutes and organizations that have participated in its construction.

Figure 3: M. Martinez giving a speech as LST Steering Committee Chair during the LST1 inauguration ceremony.

During 2018 the gamma ray group at IFAE was composed by about 15 physicists with a similar proportion of senior scientists, postdocs and Ph.D. students and shared its time between the CTA project and the MAGIC Telescopes. In addition, about 10 engineers and technicians (software, mechanics, and electronics) invested a good fraction of their time for the CTA project at IFAE. The CTA-IFAE Co-PI, J. Cortina, moved to CIEMAT by mid-2018 and was substituted by E. Moretti.

In 2018 IFAE has continued participating with high impact and visibly in CTA, keeping important responsibilities also at the management level: during 2018 IFAE has had two representatives at the highest management level of the LST Collaboration: J. Cortina has been the Co-PI of the project and M. Martinez has served as the Chair of its Steering Committee. Additionally O. Ballester has been the LST Systems Engineer, O. Blanch the Camera Coordinator, A. Moralejo the Software Coordinator and J. Cortina the Infrastructure Coordinator. All of them have been members of the Executive Board of the LST project.

In addition, in 2018 A. Moralejo has served as CTA’s Deputy Coordinator of the Analysis and Simulations working group.

M. Martinez continued as well as the leader of the 13 Spanish groups that presently constitute the CTA-Spain consortium.

CTA activities at IFAE in 2018

Figure 3: IFAE/UAB Raman LIDAR first light curves demonstrating that the performance fulfils the design goals.

For the IFAE/UAB Raman LIDAR project, during the year 2018 the LIDAR construction reached completion and first light was recorded in November, demonstrating that the chosen design fulfils the CTA requirements (see fig.3).

In what concerns mechanics, during 2018 the bogies were improved and a temporary Azimuth Locking System, a mechanical system able to lock automatically the LST in the parking position and hold it in the worst meteorological condition expected, was successfully installed right before the inauguration (see fig.4)

During the last few years, IFAE has played an important role in the CTA Analysis and Simulations working group, with the development of an independent analysis chain within the framework of the MAGIC reconstruction software (MARS).

The results from the simulations have been instrumental in the evaluation of the proposed locations for CTA north and south, and in the definition of the so-called “baseline designs” for the two arrays. Since 2017 the IFAE group led the publication “Monte Carlo Performance Studies for the Site Selection of the Cherenkov Telescope Array” (Hassan et al, Astropart. Phys. 93), in which the main results used in the site selection process are presented and was published in 2018. In addition in 2018 IFAE also led, with P. Cumani from IFAE as first author, the paper “MC Studies for the Optimisation of the CTA Layout”, recently sent for publication.

In what concerns Data, continuing its participation in the European Commission H2020 project ASTERICS, IFAE has actively contributed to the development of the first open, high-level data format for Atmospheric Cherenkov telescopes data, allowing for the first time joint analysis between different IACT experiments.

Figure 4: The Azimuth Locking System installed by the IFAE team in October 2018.

The LST Camera integration and first Cherenkov light

In 2018, the first LST camera was assembled and integrated at the IFAE workshop, in a special large dark room built for that purpose

The most important activity in 2018 has been the integration of the first LST camera in a special large dark room built at the IFAE workshop for that purpose.

By the first months of 2018 the camera enclosure, the photosensor clusters and all the needed additional components were ready or arrived at IFAE, and then for over three months all the elements were tested and integrated by an international team constituted mainly by IFAE members, together with visitors from Japan, France and Spanish groups from CIEMAT, UCM and ICE-CSIC, and led by O. Blanch (see Fig. 5) and E. Moretti.

Figure 5: The LST camera during its integration in the dark room at IFAE in summer 2018.

After completion of the integration by the end of summer 2018 the camera was prepared for shipping to La Palma (disassembling the photosensor clusters to be send in their special enclosures), reassembled back in Mirca, an onsite facility, and then installed definitely in the telescope by October. After the LST1 inauguration the Camera Team started commissioning and recorded first Cherenkov light images in December (see Fig. 6). Since mid August at least a member of the IFAE camera experts is present onsite to carry out the camera commissioning and ensure its smooth operation.

Figure 6: A first light Cherenkov event recorded at the LST1 in December 2018.