CTA: Cherenkov Telescope Array

Abelardo Moralejo

The Cherenkov Telescope Array Observatory (CTAO), currently under construction, is the next-generation facility for ground-based Gamma-Ray Astronomy, with full-sky coverage, broader energy range and a ten-fold higher flux sensitivity than current instruments like MAGIC. IFAE is one of the founding members of the CTA consortium, and has co-led the construction of the Large-Sized Telescopes (LSTs) for the CTA-North observatory, located on the island of La Palma.

Activities in 2023

IFAE has contributed key elements of the LST mechanics (bogies for the undercarriage and its locking system) and the cameras (power distribution, trigger electronics and control software). The cameras for all four LSTs of CTA-North have been integrated and validated at the premises of the IFAE technical division prior to their shipping to the IAC (Instituto de Astrofísica de Canarias) in Tenerife, where they undergo final tests before final installation. The last of the four cameras was delivered to IAC in May 2023. IFAE is also contributing to the development of a Raman LIDAR, an auxiliary instrument to be deployed in the CTA-North site for the monitoring of the atmospheric conditions during observations.

Figure 3: LST-1 during observations (Credit: CTAO gGmbH)

Figure 4 : Status of the construction of LST-4, with the rail and bogies for the azimuthal movement already in installed (Credit: LST collaboration)

The first telescope of CTA-North, LST-1, was completed in 2019, and has been performing (part-time) scientific observations for the past four years, already during its commissioning phase. The civil works for the remaining three LSTs were completed during 2023, and construction of the telescope structures is underway.

Besides its contribution to the construction of the observatory, the IFAE team plays a strong role in the development of the LST data analysis software, and has led the validation of the telescope performance through observations of the Crab Nebula, the standard candle of astronomy in the very-high energy (VHE) band.

Observations of the Crab Nebula and Pulsar with the first Large-sized Telescope of the Cherenkov Telescope Array

This effort culminated with the publication in October 2023 of the paper “Observations of the Crab Nebula and Pulsar with the Large-sized Telescope Prototype of the Cherenkov Telescope Array” (H. Abe et al 2023 ApJ 956 80), which presents the telescope’s capabilities in terms of flux sensitivity, angular and energy resolution, and validates the Monte Carlo simulations required for data analysis. The paper is based on 34 hours of Crab observations carried out between November 2020 and March 2022, on which, besides the bright emission from the Nebula, a clear (11-𝜎) detection of the faint, soft emission from the central pulsar was achieved, confirming the excellent performance of LST-1 in the lower-energy end of the VHE band. This publication is a significant milestone in the scientific exploitation of LST-1, as a necessary reference for the publication of the observations of many other targets performed over the past years with LST-1, among which the recent detection of the active galaxy OP 313, the most distant VHE emitter of its class known to date (https://www.astronomerstelegram.org/?read=16381).

Figure 1: Spectral energy distribution of the Crab Nebula obtained with 34 hours of observation with the LST-1 telescope collected in 2020 - 2022 (ApJ 956 80). Results from 7.4 years of observations with the Fermi-LAT space telescope are shown for comparison.

We note that, while as a standalone telescope LST-1 is already a competitive instrument among its previous-generation predecessors, the array of four LSTs operating in stereoscopic mode (expected for 2026) will outperform all the current VHE telescope arrays.