[Paper Review] Observing the sky at extremely high energies with the Cherenkov Telescope Array: Status of the GCT project

Sol, H.+2 more|arXiv (Cornell University)|Jan 1, 2017

Astrophysics and Cosmic Phenomena3 references4 citations

TL;DR

The GCT project presents the status of the Gamma-ray Cherenkov Telescope, a 4-meter Schwarzschild-Couder telescope designed for the Cherenkov Telescope Array (CTA) to observe the sky at extremely high energies (5–300 TeV). It achieved first light in 2015 and successfully completed a 2017 observing campaign, validating its design and performance, with the CHEC-S camera prototype now under advanced testing for future deployment in the CTA southern array.

ABSTRACT

The Cherenkov Telescope Array is the main global project of ground-based gamma-ray astronomy for the coming decades. Performance will be significantly improved relative to present instruments, allowing a new insight into the high-energy Universe [1]. The nominal CTA southern array will include a sub-array of seventy 4 m telescopes spread over a few square kilometers to study the sky at extremely high energies, with the opening of a new window in the multi-TeV energy range. The Gamma-ray Cherenkov Telescope (GCT) is one of the proposed telescope designs for that sub-array. The GCT prototype recorded its first Cherenkov light on sky in 2015. After an assessment phase in 2016, new observations have been performed successfully in 2017. The GCT collaboration plans to install its first telescopes and cameras on the CTA site in Chile in 2018-2019 and to contribute a number of telescopes to the subsequent CTA production phase.

Motivation & Objective

• To develop a next-generation ground-based gamma-ray telescope for the Cherenkov Telescope Array (CTA) to explore the extremely high-energy (EHE) sky above 10 TeV.
• To validate the feasibility and performance of the novel Schwarzschild-Couder optical design for small-size telescopes (SSTs) in gamma-ray astronomy.
• To demonstrate the technical readiness of the GCT prototype, including its telescope structure, pointing system, and the CHEC-M and CHEC-S cameras, for integration into the CTA southern array.
• To prepare for the pre-production and production phases of CTA by delivering at least 25 GCT telescopes as in-kind contributions to the observatory.

Proposed method

• Utilization of a dual-mirror Schwarzschild-Couder optical design to achieve a large field of view (8°) and compact focal length, enabling a lightweight and stable telescope structure.
• Implementation of a high-speed, high-resolution camera (CHEC) with 2048 pixels using Multianode Photo-multiplier Tubes (MAPMTs) and Silicon Photo-multipliers (SiPMs), sampled at 1 GSa/s for full-waveform digitization.
• Deployment of the GCT prototype at Meudon, France, equipped with the CHEC-M and CHEC-S camera prototypes, to conduct ground-based testing and sky observations.
• Use of Monte Carlo simulations (CORSIKA and sim_telarray) to model atmospheric showers and compare with on-sky data for instrument validation.
• Remote operation of the telescope and camera during the 2017 observing campaign, enabling real-time data acquisition and system diagnostics.
• Integration of predictive models and real-time monitoring (e.g., guide cameras) to achieve sub-arcsecond pointing precision, with a target of <7” rms post-calibration.

Experimental results

Research questions

• RQ1 Can a Schwarzschild-Couder telescope design achieve the required angular resolution and sensitivity for detecting very high-energy gamma rays above 10 TeV?
• RQ2 Does the GCT prototype successfully record and reconstruct Cherenkov light from atmospheric showers, confirming its performance against simulation benchmarks?
• RQ3 Can the CHEC-S camera, using SiPMs and advanced ASICs (TARGET 5), overcome limitations of the earlier CHEC-M prototype in dynamic range and timing resolution?
• RQ4 What is the performance of the GCT's pointing system under real sky conditions, and can it achieve the required <7” rms pointing accuracy?
• RQ5 How do on-sky data from the 2017 campaign compare with Monte Carlo simulations in terms of shower image parameters such as length and width?

Key findings

• The GCT prototype successfully recorded its first Cherenkov light on sky on November 26, 2015, marking the first time a Schwarzschild-Couder telescope detected atmospheric showers.
• The 2017 observing campaign successfully registered several thousand cosmic showers under various sky conditions, both with and without source tracking, confirming system stability and functionality.
• On-sky data showed good agreement with Monte Carlo simulations, particularly in the distribution of shower images in the 'length-width' plane, validating the instrument's response and reconstruction capabilities.
• The GCT prototype achieved a pointing precision better than 210” before calibration, with a target of <7” rms post-calibration, demonstrating high mechanical stability.
• The CHEC-S camera prototype, featuring improved ASICs and SiPM technology, is now under intensive laboratory testing and is expected to surpass the performance of the CHEC-M prototype in dynamic range and timing resolution.
• The final GCT design for the CTA SST sub-array has been optimized for mass production, with the GCT-01 telescope expected to be launched soon for installation at the CTA site in Chile.