Littérature scientifique sur le sujet « Astri-horn »

We report on the first detection of very high-energy gamma-ray emission from the Crab Nebula by a Cherenkov telescope in dual-mirror Schwarzschild-Couder (SC) configuration. This result has been achieved by means of the 4 m 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, which allows us to implement large cameras with a large field of view, with small-size pixel sensors and a high level of compactness. The curved focal plane of the ASTRI camera is covered by silicon photo-multipliers, managed by an unconventional front-end electronic system that is 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. These 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 between on- and off-axis source exposure. The camera system was still under commission 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 5.4σ above an energy threshold of ∼3 TeV. This result provides an important step toward the use of dual-mirror systems in Cherenkov gamma-ray astronomy. A pathfinder mini-array based on nine ASTRI-like telescopes with a large field-of-view is in the course of implementation.

AbstractASTRI-Horn is an Imaging Atmospheric Cherenkov Telescope characterized by a dual-mirror optical system with a primary mirror diameter of 4.3 m and a curved focal surface covered by silicon photomultiplier (SiPM) sensors managed by an innovative fast front-end electronics. ASTRI-Horn is installed in Italy at the INAF “M.C. Fracastoro” observing station (Mount Etna, Italy); it is the prototype of nine similar telescopes forming the ASTRI MiniArray that will be installed at the Teide Astronomical Observatory, in Tenerife (Canary Islands, Spain). In the ASTRI-Horn camera, the output signals from SiPMs are AC coupled to the front-end electronics stopping any slow varying signals. However, the random arrival of the night sky background photons produces fast fluctuations in the signal that the electronics is able to detect. The noise generated by this effect is proportional to the level of the diffuse night sky background. In this work, we present the analysis of the background data in ASTRI-Horn observations during the period December 2018–March 2019, using images of triggered showers. We compare the results relative to 2018 December 7-8 and 2019 March 6-7 nights with the contemporary night sky background fluxes measured by UVscope. This is a small auxiliary instrument mounted on the external structure of the ASTRI-Horn telescope and devoted to the night sky background evaluation in the UV band. A strong correlation between the considered data was detected. This correlation can be a diagnostic tool to assure the proper behavior of the ASTRI-Horn camera in view of the ASTRI MiniArray implementation. ASTRI-Horn is also equipped with the Variance technique able to sample the level of the pixel signals in absence of showers with an high rate. The method presented in this paper, based on shower images, is a new approach that has never been investigated until now. It does not substitute the Variance, that will the baseline for the background evaluation after exhaustive testings, but it is complementary to it when Variance data are available. This is the only one method working very well, that can be applied whenever the standard Variance method is not operative.

AbstractUVscope is an instrument, based on a multi-pixel photon detector, developed to support experimental activities for high-energy astrophysics and cosmic ray research. The instrument, working in single photon counting mode, is designed to directly measure light flux in the wavelengths range 300-650 nm. The instrument can be used in a wide field of applications where the knowledge of the nocturnal environmental luminosity is required. Currently, one UVscope instrument is allocated onto the external structure of the ASTRI-Horn Cherenkov telescope devoted to the gamma-ray astronomy at very high energies. Being co-aligned with the ASTRI-Horn camera axis, UVscope can measure the diffuse emission of the night sky background simultaneously with the ASTRI-Horn camera, without any interference with the main telescope data taking procedures. UVscope is properly calibrated and it is used as an independent reference instrument for test and diagnostic of the novel ASTRI-Horn telescope.

In the study of Very High-Energy (VHE) astrophysical phenomena the next generation of Imaging Atmospheric Cherenkov Telescopes (IACT) will play a key role thanks to specific ground-based astronomical observations. In this context, the ASTRI project developed a novel instrument endowed with a Schwarzschild-Couder dual-mirror optical configuration (that has never been adopted before in gamma-ray astronomy) and a dedicated Cherenkov camera entirely designed by the Istituto Nazionale di Astrofisica (INAF) based on SiPM sensors. The prototype telescope ASTRI– Horn is located in Italy and carried out successfully in 2019 the techno- logy validation phase, paving the way for the realization of the MiniAr- ray: 9 identical telescopes working in stereoscopic mode to be installed in Tenerife (Canary Islands) within the next three years. However, several issues related to the pointing performances emerged during operations with ASTRI–Horn. Actually, the pointing calibration is generally a critical aspect for Cherenkov telescopes, as their cameras are designed for the detection of nanosecond atmospheric flashes rather than for imaging the starfield and, consequently, it is impossible to use the standard astrometry of the focal plane. Furthermore, in the case of ASTRI, the compactness of the mechanical structure prevents from installing an auxiliary monitoring camera sharing the same optical system of the telescope. Despite these difficulties, the optimization of the pointing performances is crucial for ensuring the scientific accuracy of the whole system. The present PhD thesis aims at the development and validation of new astrometric techniques for the pointing calibration of the ASTRI telescope exploiting the so-called Variance method, a statistical algorithm implemented in the Cherenkov camera electronic board. Thanks to the Variance, the AS- TRI telescope is endowed with an ancillary output owning the potentiality to image the stellar component of the night sky background in the Field of View (FoV), with a quite coarse angular resolution (~11°, corresponding to the pixel size of the Cherenkov camera), but a relatively good sensitivity for an IACT (visual magnitude limit ~7). As we discuss in this document the Variance constitutes a unique opportunity for enhancing the pointing performances of the telescope, and we demonstrate that our procedures offer a chance to reach the critical accuracy level required for achieving the scientific objectives of the ASTRI project. Unfortunately, in this period the COVID-19 pandemic and other accidental events, heavily delayed the maintenance operations on the ASTRI– Horn telescope, and up to now it is still impossible to make new observations dedicated to the validation of our procedures, hence only data taken in previous months were used. As in any other experimental activity, new data taken on purpose would have considerably facilitated our work, but due to the present situation, we focused our attention on Variance data available in the ASTRI archive that has never been explored before. The resulting work represents the first complete and detailed analysis of the Variance method together with its numerous unexplored applications. Our custom astrometry techniques allowed us to reveal that ASTRI–Horn was affected by two kinds of systematic errors, that we characterized and measured for the first time. The experience gained with archive data allowed us to understand how to apply our routines for calibrating the incoming ASTRI MiniArray, indicating an effective strategy to match the crucial requirement for the pointing accuracy. The resulting procedure has already been inserted into the calibration plan of the MiniArray and its Online Observation Quality System (OOQS). The structure of the present document is articulated in eight chapters and three appendices, whose content can be summarized as follows. Chapter 1 presents the status of the art in VHE astrophysics, focusing on the observational features of cosmic rays and gamma rays, together with a description of the main open questions in this research field. Chapter 2 is dedicated to IACTs, presenting their history and operating principles, and introducing the major examples of instruments currently in activity worldwide. Chapter 3 focuses on the ASTRI project, presenting both the prototype telescope ASTRI–Horn and the incoming observatory of the MiniArray. In particular, it is reported a detailed description of the most relevant sub-systems for this thesis: the camera, the optical scheme, and the pointing strategy. Chapter 4 goes into the details of the Variance method. A technical description of its functioning at the electronic level is provided at first, while the core of the chapter is dedicated to our routines for the production of sky images and their calibration. Chapter 5 reports specific tools and procedures that we developed for the analysis of Variance images: the astrometric calibration, the de-convolution of the star signal, and the transformation function to correct the artifacts introduced by the geometric arrangement of the pixels. Chapter 6 describes the algorithm to assess the alignment of the Cherenkov camera to the optical axis of the telescope exploiting the apparent rotation of the FoV during long observing runs in tracking mode. Chapter 7 shows a custom procedure for the star identification developed on purpose for Variance images (as it is impossible to adopt the standard astrometry software for their analysis) allowing to monitor in real-time the actual pointing direction of the telescope. Chapter 8 contains the concluding remarks. It summarizes the main results achieved in this thesis, highlighting their importance but also some limitations and suggesting further improvements. Future perspectives of this work are briefly presented at last, with particular attention to its implementation on the incoming ASTRI MiniArray. At the end of this document, three appendices report additional/complementary material concerning respectively metrological techniques for the inspection of shape and reflectivity of primary mirror segments (A), more details about the software developed for this thesis and the access to it (B), and the massive activity of outreach and education carried out during the doctoral period in the field of Cherenkov astronomy (C).