First detection of the Crab Nebula at TeV energies with a Cherenkov telescope in a dual-mirror Schwarzschild-Couder configuration: the ASTRI-Horn telescope

Abstract

We report on the first detection of very high-energy (VHE) gamma-ray emission from the Crab Nebula by a Cherenkov telescope in dual-mirror Schwarzschild-Couder (SC) configuration. The result has been achieved by means of the 4 m size ASTRI-Horn telescope, operated on Mt. Etna (Italy) and developed in the context of the Cherenkov Telescope Array Observatory preparatory phase. The dual-mirror SC design is aplanatic and characterized by a small plate scale, allowing us to implement large field of view cameras with small-size pixel sensors and a high compactness.The curved focal plane of the ASTRI camera is covered by silicon photo-multipliers (SiPMs), managed by an unconventional front-end electronics based on a customized peak-sensing detector mode. The system includes internal and external calibration systems, hardware and software for control and acquisition, and the complete data archiving and processing chain. The observations of the Crab Nebula were carried out in December 2018, during the telescope verification phase, for a total observation time (after data selection) of 24.4 h, equally divided into on-and off-axis source exposure. The camera system was still under commissioning and its functionality was not yet completely exploited. Furthermore, due to recent eruptions of the Etna Volcano, the mirror reflection efficiency was reduced. Nevertheless, the observations led to the detection of the source with a statistical significance of 5.4 σ above an energy threshold of ∼3 TeV. This result provides an important step towards the use of dual-mirror systems in Cherenkov gamma-ray astronomy. A pathfinder mini-array based on nine large field-of-view ASTRI-like telescopes is under implementation.

The ASTRI-Horn Dual-Mirror Small-Size Cherenkov Telescope: recent updates, first results and outlook

“…The analysis of these scans indicated the camera's working point for the detection of Cherenkov light to be at ∼ 11 photoelectrons (phe), meaning that the trigger rate in the region dominated by air-shower events was lower than expected from previous measurements in the laboratory, due to a reduced telescope efficiency at that time that was associated with the mirror degradation. On the other hand, the trigger threshold was found to be higher due to the local night sky background being higher than expected [7,15].…”

“…In December 2018, 12.4 hours of gamma-ray data on the Crab Nebula and 12.0 hours of dedicated off-target data were collected, respectively, resulting in a detection of this well-known VHE emitter above ∼ 3 TeV [15], despite the reduced efficiency of the telescope as well as a non-optimal tuning of the camera.…”

UVscope and its application aboard the ASTRI-Horn telescope

“…These conditions, mainly extreme for the ASTRI-Horn camera, allowed scientific acquisitions only for a portion of the foreseen useful nights. Nevertheless, the period was very fruitful for ASTRI-Horn that obtained its first detection of the Crab Nebula (R.A. J 2000 =5h34m31.94s and Dec J 2000 =+22 • 00'52.2") [15]. Figures 12,13 and 14 show the diffuse NSB flux in the wavelength band of UVscope as obtained by the analysis of the data acquired contemporarily with ASTRI-Horn camera; superimposed is indicated the ASTRI-Horn acquisition run number and the telescope pointing (Zeta Tauri is located at about 1.5 • far from the Crab Nebula).…”

The ASTRI Mini-Array Core Science Program

“…, 8]. A few telescope prototypes were developed in the recent years, among them the ASTRI-Horn dual-mirror, Schwarzschild-Couder telescope [9], currently operating on Mount Etna in Sicily (Italy), which obtained the firstlight optical qualification by means of observation of Polaris, using a dedicated optical camera [10], and the first detection of very high-energy γ-ray emission from the Crab Nebula by a Cherenkov telescope in dual-mirror Schwarzschild-Coudé (SC) configuration [11]. [12].…”