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 in its full configuration, CTA shall be the world’s largest and most sensitive high-energy gamma-ray observatory.

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 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.

Introduction: Progress of the CTA project in 2020

In spite of the extremely difficult situation worldwide due to the covid19 pandemic, in 2020 the CTA project kept steadily progressing towards the constitution of an ERIC (European Research Infrastructure Consortium) structure for the CTA Observatory. The main highlights were the approval of a detailed and comprehensive Cost Book by the CTA Observatory Council, and the constitution of a Board of Government Representatives (BGR) that has gathered a Money-Matrix with the funding commitments for the construction of CTA. The comparison of these two documents has unveiled the need for some re-dimensioning of the original plans for CTA, that is being currently reconsidered.

During 2020 the gamma ray group at IFAE was composed by about 15 physicists with similar proportion of senior scientists, postdocs and PhD students and shared its time between the CTA project and the MAGIC Telescopes. In addition, about 8 engineers and technicians (software, mechanics and electronics) invested a good fraction of their time for the CTA project at IFAE.

Along 2020 IFAE has held important responsibilities at the highest management level of the LST Collaboration: M. Martinez has served as the Chair of its Steering Committee, O. Ballester has been the LST Systems Engineer, O. Blanch the Camera Coordinator and A. Moralejo the Software Coordinator. All them have been members of the Executive Board of the LST project.

The Barcelona Raman LIDAR project

For the Barcelona Raman LIDAR project, during the year 2020 the project became a truly international endeavour with the start of the participation of the LIDAR group from the University of Nova Gorizia in Slovenia, in addition to the long-standing participation of Padova University in Italy. The commissioning of the LIDAR in Barcelona was deemed completed after being able to re-aluminize its 1.8m mirror in the ZAOT Italian company during November in spite of the travel restrictions and additional difficulties due to the covid-19 pandemic (see fig 2). The results obtained at IFAE with the new mirrors were really good (see fig 3) and suggested that the LIDAR was ready for starting onsite tests at ORM. As a consequence, in December, the LIDAR was shipped to La Palma (see fig 4) for one-year pathfinder testing period at the ORM within the LST1 site. This one-year test during 2021 shall demonstrate that the LIDAR on-site performance and endurance do fulfil the CTA requirements.

Figure 1: The LIDAR mirror before and after re-aluminization on November 2020.

Figure 1: Plots showing the LIDAR performance achieved after mirror aluminization during its last commissioning nigh at IFAE

Figure 1: The LIDAR wrapped in the truck for its transport to ORM in Dec 2020.

The LST project

In what concerns the LST project, during 2020 the commissioning of the LST1 has kept steadily progressing in spite of the periods of lockdown of the ORM, the travel restrictions for the shifters and for the technical teams, and all the additional difficulties related to the covid19 pandemic. In addition, in February the LST1 telescope suffered the effects of the severe calima storm that reached windspeeds over 160 Km/h, and its camera lid was damaged and its repair took a rather long time due to the difficult covid19 pandemic. In those very difficult conditions, it was an outstanding effort, specially by IFAE members (3 out of the 4 first shift-crew members for LST1+MAGIC after the end of the ORM lockdown were from IFAE), what allowed the LST1 commissioning to keep going.

In addition, the activities to build part of the cameras and integrate them for the next 3 LSTs started. They were conducted in the framework of a bidding awarded to an “Union Temporal de Empresas” formed by IFAE and IEEC. Both members of the gamma ray group and engineers are involved, and the activity will intensify in the next two years when three cameras need to be delivered

Through 2020 the IFAE team coordinated and contributed to the development of the analysis tools for the LST telescope, with a focus on the camera calibration, data quality checks and production of high-level data (gamma candidates event lists) for spectral analysis.

An important highlight showing the overall progress in the operation of the LST1 and in its data analysis, is that the runs between January and February 2020 allowed an unambiguous detection of the Crab Pulsar, the neutron star at the centre of the Crab Nebula (see fig 1).

Figure 1: Phasogram of Crab Pulsar as measured by the LST-1. The pulsar is known to emit pulses of gamma rays during phases P1 and P2. The shown significance is calculated considering source emission from those phases (in red) and background events from phases in grey.

In what concerns Data, after the end of the European Commission H2020 project ASTERICS, IFAE has kept actively contributing to the development of the first open, high-level data format for Atmospheric Cherenkov telescopes data, now within the EU Project ESCAPE. Within this framework, an important activity by IFAE students and postdocs has been converting data to the DL3 format and analyse those files with CTA software (GammaPy).