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. CTA will be deployed in two sites, CTA-North in the Roque de los Muchachos observatory in the island of La Palma, and CTA-South in the Atacama desert, near the ESO Paranal observatory. With its unprecedented sensitivity, CTA will contribute to the understanding of the most extreme astrophysical environments in the universe, in which particles can be accelerated to ultra-high energies and produce gamma radiation via their interaction with ambient matter or radiation. CTA will also search for signatures of dark matter annihilation, and test Einstein’s theory of relativity by studying the propagation of very-high energy photons across cosmological distances.

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

IFAE is part of the CTA consortium since its foundation, with efforts focused on three fronts: the cameras of the CTA large-sized telescopes (LSTs) which target the tens-of-GeV energy band, the development of instrumentation for atmospheric monitoring, and the development of the data analysis software. In 2021 the CTA IFAE group was composed of around 10 physicists including senior scientists, postdocs and PhD students who typically also contributed to other projects of the gamma-ray group (mainly MAGIC). In addition, about 8 engineers and technicians (software, mechanics and electronics) invested a fraction of their time for the CTA project at IFAE.

Progress of the CTA project in 2021

On 24 June 2021, the Board of Governmental Representatives (BGR) approved the CTA Observatory’s (CTAO’s) Cost Book and Scientific & Technical Description. The configuration of the telescope arrays at the two sites for the first construction phase, named “Alpha Configuration’’ was defined. This configuration includes 4 LSTs and 9 Medium-Sized Telescopes (MSTs) in CTA-North, and 14 MSTs and 37 Small-Sized Telescopes (SSTs) in CTA-South.

LST-1, the first LST built in the CTA-North site, has been performing regular sky observations since December 2019. Its commissioning phase, not yet officially completed, has been hampered by difficulties associated with the pandemic, and, in 2021, also by the eruption of the Cumbre Vieja volcano, which prevented the operation of the telescope for nearly four months. The telescope has not suffered any damage as a result of the eruption. Through 2021 the IFAE team maintained its leading role in the commissioning of the camera control software and its trigger system, and in the development of the official LST data analysis pipeline to transform raw camera data into lists of gamma-ray candidate events prepared for the higher-level analysis. IFAE has also kept actively contributing to the development of open data formats and analysis tools for Gamma-ray Astronomy, within the EU Project ESCAPE. During 2021 we have pioneered the integration of our analysis software contributions and our GammaHub iterative, multi-instrument, big-data analysis tool into the ESFRI Science Analysis Platform, thus taking an important leap towards the achievement of the project objectives.

In what concerns the construction of the next three LSTs of CTA-North, IFAE contributes the camera trigger and power systems, and will lead and host at the IFAE workshop the integration of the three remaining LST cameras throughout 2022. The integration of the LST-2 camera started in November 2021 and is close to completion as of March 2022.

First physics results of LST-1

IFAE is playing a central role in the early scientific exploitation of LST-1. Results of several source observations (among which the Crab nebula and its pulsar, the Galactic center, and six different active galaxies) were presented for the first time in conferences in 2021, and publications in peer-reviewed journals are in preparation. The observations of the Crab Nebula (the standard candle of gamma-ray astronomy) show that the telescope performs according to the expectations from Monte Carlo simulations for a standalone LST, and can obtain energy spectra consistent with those from well-established previous generation instruments like MAGIC (see Fig. 1). The LST Collaboration has also developed a programme for the observation of transient gamma-ray sources, like active galaxies, in the context of which we delivered the first Astronomical Telegram from CTA (ATEL #14783), announcing the detection by LST-1 of the BL Lac galaxy in an elevated gamma-ray flux state on July 10, 2021. The LST-1 observations were triggered by an increase in the brightness of the source in the optical band reported by the {\it Whole Earth Blazar Telescope}. This result clearly demonstrates the ability of LST-1 not only to perform quick follow-up observation of transient sources, but also to produce robust results and alert the astronomical community in a short time.

Figure 1: Early physics results of LST-1 observations. Gamma-ray spectrum of the Crab Nebula obtained with a 3.5 hours of observation with LST-1 performed in November 2020. A spectrum obtained with MAGIC is shown for comparison.

Figure 1: Early physics results of LST-1 observations. Sky map (number of recorded gamma-like events per unit solid angle) of the region around the active galaxy Mrk 421observed in 2020. Credit: LST collaboration.

The Barcelona Raman LIDAR

The Barcelona Raman LIDAR (BRL) is a pathfinder of the atmospheric monitoring instrumentation for CTA, developed by IFAE in collaboration with the UAB IEEC-CERES (Institute of Space Studies of Catalonia - Center of Space Studies and Research), the university of Padua (Italy) and the university of Nova Gorica (Slovenia). The instrument aims at measuring the molecular profile and the aerosols profile of the atmosphere in the range relevant for the development of the particle showers detected by the CTA telescopes. This instrument will allow to reduce the systematic uncertainties of the CTA observations, particularly in the reconstruction of the photon energies. During 2021 the BRL, installed in the CTA-North site, was operated on a monthly basis, on days around the full moon. IFAE participated in these on-site data taking shifts and in the analysis of the BRL data. The possibility of performing remote and semi-remote observations with the BLR, with minimal intervention of on-site personnel, was successfully tested. Finally, during the Cumbre Vieja volcano eruption, the observations with the BLR allowed the detection of the volcanic ash plume over the ORM (see Fig. 2). The measured LIDAR ratio and Ångström coefficients determined that the plume was made of particles with large diameters, typical for these events.

Figure 3: Left: vertical profile of the different atmospheric layers obtained by the BRL on September 22. The peak between 1.2 and 1.6 km above the telescope corresponds to radiation backscattered by the volcanic dust plume, while the higher peak is interpreted as a cloud with typical characteristics for La Palma at those altitudes. Data from the MAGIC LIDAR system located next to the LST-1 was provided as a courtesy of the MAGIC Collaboration for comparison. Center: Extinction coefficients. Right: Retrieved LIDAR ratios. Credit: BRL team.