ASTRI for the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) will be the largest ground-based observatory operating in the very-high-energy gamma-ray (20 GeV - 300 TeV) range. It will be based on more than one hundred telescopes, located at two sites (northern and southern hemispheres). The energy coverage, in the southern CTA array, will extend up to hundreds of TeV thanks to 70 small size telescopes (SST), with primary mirrors of about 4 meters in diameter and large field of view of the order of 9 degrees. It is proposed that one of the first sets of precursors for the CTA SSTs array will be represented by nine ASTRI telescopes. Their prototype, named ASTRI SST-2M, is installed in Italy. It is currently completing the overall commissioning before entering the science verification phase that will performed observing bright TeV sources as Crab Nebula, Mrk421 and Mrk 501 cross-checking the prototype performance with the Monte Carlo predictions. ASTRI telescopes are characterized by a dual-mirror optical design based on the Schwarzschild- Couder (SC) configuration. The focal-plane camera is curved in order to fit the ideal prescription for the SC design and the sensors are small size silicon photomultipliers read-out by a fast front-end electronics. The telescope prototype installed in Italy, has been developed by the Italian National Institute for Astrophysics, INAF, following an end-to-end approach that comprises all aspects from the design, construction and implementation of the entire hardware and software system to the final scientific products. All parts of the system have been designed to comply with the CTA requirements. A collaborative effort, addressed to the implementation of the first ASTRI telescopes for the CTA southern site, is now on-going led by INAF with the Universidade de Sao Paulo (Brazil), the North-West University (South Africa) and the Italian National Institute for Nuclear Physics.Comment: All CTA contributions at arXiv:1709.0348

The EUSO Data Simulation and Analysis Tree

The "Extreme Universe Space Observatory - EUSO" is the first Space mission devoted to the exploration of the outermost bounds of the Universe through the investigation of the Extremely-High Energy Cosmic Rays, EECR, using the Earth atmosphere as a giant detector. The objective is to obtain a detailed description of the Cosmic Ray spectrum beyond 5×1019 eV together with a map of the arrival directions. EUSO will detect EECR looking at the streak of fluorescence light produced when such a particle interacts with the atmosphere. The signal will be detected after its propagation upward from the dark Earth atmosphere to the EUSO telescope accommodated, as external payload, on the International Space Station. EUSO is a mission of the European Space Agency ESA, and it is now completing the "Phase A" study with a goal for a three year mission starting in 2010. The various peculiarities of the EUSO space-based observational approach imply a dedicated effort for the evaluation of the expected features of the detected signals and for the reconstruction of its space-time development, energy and composition, namely from simulation and data analysis point of view. A complete software infrastructure named ESAF (EUSO Simulation and Analysis Framework) is under development suitable for event simulation, detector response simulation, event reconstruction and scientific data analysis

Calibration of the ASTRI SST-2M Prototype using Muon Ring Images

The study of ring images generated from high-energy muons is a very useful tool for the performance monitoring and calibration of any Imaging Atmosphere Cherenkov Telescope. Isolated muons travelling towards the telescope light collector system produce characteristic Cherenkov ring images in the focal plane camera. Since the geometry and the distribution of light deployed onto the camera can be easily reconstructed analytically for a muon of given energy and direction, muon rings are a powerful tool for monitoring the behaviour of crucial properties of an imaging telescope such as the point-spread-function and the overall light collection efficiency. In this contribution we present the possibility of using the analysis of muon ring images as calibrator for the ASTRI SST-2M prototype point spread function.Comment: In Proceedings of the 33rd International Cosmic Ray Conference (ICRC2013), Rio de Janeiro (Brazil). All ASTRI contributions at arXiv:1307.463

All Sky Camera, LIDAR and Electric Field Meter: auxiliary instruments for the ASTRI SST-2M prototype

ASTRI SST-2M is the end-to-end prototype telescope of the Italian National Institute of Astro- physics, INAF, designed to investigate the 10-100 TeV band in the framework of the Cherenkov Telescope Array, CTA. The ASTRI SST-2M telescope has been installed in Italy in September 2014, at the INAF ob- serving station located at Serra La Nave on Mount Etna. The telescope is foreseen to be completed and fully operative in spring 2015 including auxiliary instrumentation needed to support both operations and data anal- ysis. In this contribution we present the current status of a sub-set of the auxiliary instruments that are being used at the Serra La Nave site, namely an All Sky Camera, an Electric Field Meter and a Raman Lidar devoted, together with further instrumentation, to the monitoring of the atmospheric and environmental conditions. The data analysis techniques under development for these instruments could be applied at the CTA sites, where similar auxiliary instrumentation will be installed.Comment: Proceedings of the 2nd AtmoHEAD Conference, Padova (Italy) May 19-21, 201

Volcanoes muon imaging using Cherenkov telescopes

A detailed understanding of a volcano inner structure is one of the key-points for the volcanic hazards evaluation. To this aim, in the last decade, geophysical radiography techniques using cosmic muon particles have been proposed. By measuring the differential attenuation of the muon flux as a function of the amount of rock crossed along different directions, it is possible to determine the density distribution of the interior of a volcano. Up to now, a number of experiments have been based on the detection of the muon tracks crossing hodoscopes, made up of scintillators or nuclear emulsion planes. Using telescopes based on the atmospheric Cherenkov imaging technique, we propose a new approach to study the interior of volcanoes detecting the Cherenkov light produced by relativistic cosmic-ray muons that survive after crossing the volcano. The Cherenkov light produced along the muon path is imaged as a typical annular pattern containing all the essential information to reconstruct particle direction and energy. Our new approach offers the advantage of a negligible background and an improved spatial resolution. To test the feasibility of our new method, we have carried out simulations with a toy-model based on the geometrical parameters of ASTRI SST-2M, i.e. the imaging atmospheric Cherenkov telescope currently under installation onto the Etna volcano. Comparing the results of our simulations with previous experiments based on particle detectors, we gain at least a factor of 10 in sensitivity. The result of this study shows that we resolve an empty cylinder with a radius of about 100 m located inside a volcano in less than 4 days, which implies a limit on the magma velocity of 5 m/h.Comment: 21 pages, 21 figures, in press on Nuclear Inst. and Methods in Physics Research, A. Final version published online: 3-NOV-201

Tools and Procedures for the CTA Array Calibration

The Cherenkov Telescope Array (CTA) is an international initiative to build the next generation ground-based very-high-energy gamma-ray observatory. Full sky coverage will be assured by two arrays, one located on each of the northern and southern hemispheres. Three different sizes of telescopes will cover a wide energy range from tens of GeV up to hundreds of TeV. These telescopes, of which prototypes are currently under construction or completion, will have different mirror sizes and fields-of-view designed to access different energy regimes. Additionally, there will be groups of telescopes with different optics system, camera and electronics design. Given this diversity of instruments, an overall coherent calibration of the full array is a challenging task. Moreover, the CTA requirements on calibration accuracy are much more stringent than those achieved with current Imaging Atmospheric Cherenkov Telescopes, like for instance: the systematic errors in the energy scale must not exceed 10%.In this contribution we present both the methods that, applied directly to the acquired observational CTA data, will ensure that the calibration is correctly performed to the stringent required precision, and the calibration equipment that, external to the telescopes, is currently under development and testing. Moreover, some notes about the operative procedure to be followed with both methods and instruments, will be described. The methods applied to the observational CTA data include the analysis of muon ring images, of carefully selected cosmic-ray air shower images, of the reconstructed electron spectrum and that of known gamma-ray sources and the possible use of stereo techniques hardware-independent. These methods will be complemented with the use of calibrated light sources located on ground or on board unmanned aerial vehicles.Comment: All CTA contributions at arXiv:1709.0348

Pre-selecting muon events in the camera server of the ASTRI telescopes for the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) represents the next generation of ground based observatories for very high energy gamma ray astronomy. The CTA will consist of two arrays at two different sites, one in the northern and one in the southern hemisphere. The current CTA design foresees, in the southern site, the installation of many tens of imaging atmospheric Cherenkov telescopes of three different classes, namely large, medium, and small, so defined in relation to their mirror area; the northern hemisphere array would consist of few tens of the two larger telescope types. The telescopes will be equipped with cameras composed either of photomultipliers or silicon photomultipliers, and with different trigger and read-out electronics. In such a scenario, several different methods will be used for the telescopes' calibration. Nevertheless, the optical throughput of any CTA telescope, independently of its type, can be calibrated analyzing the characteristic image produced by local atmospheric highly energetic muons that induce the emission of Cherenkov light which is imaged as a ring onto the focal plane if their impact point is relatively close to the telescope optical axis. Large sized telescopes would be able to detect useful muon events under stereo coincidence and such stereo muon events will be directly addressed to the central CTA array data acquisition pipeline to be analyzed. For the medium and small sized telescopes, due to their smaller mirror area and large inter-telescope distance, the stereo coincidence rate will tend to zero; nevertheless, muon events will be detected by single telescopes that must therefore be able to identify them as possible useful calibration candidates, even if no stereo coincidence is available. This is the case for the ASTRI telescopes, proposed as pre-production units of the small size array of the CTA, which are able to detect muon events during regular data taking without requiring any dedicated trigger. We present two fast algorithms to efficiently use uncalibrated data to recognize useful muon events within the single ASTRI camera server while keeping the number of proton induced triggers as low as possible to avoid saturating the readout budget towards the central CTA data analysis pipeline. <P /

Calibration of the Cherenkov Telescope Array

The construction of the Cherenkov Telescope Array is expected to start soon. We will present the baseline methods and their extensions currently foreseen to calibrate the observatory. These are bound to achieve the strong requirements on allowed systematic uncertainties for the reconstructed gamma-ray energy and flux scales, as well as on the pointing resolution, and on the overall duty cycle of the observatory. Onsite calibration activities are designed to include a robust and efficient calibration of the telescope cameras, and various methods and instruments to achieve calibration of the overall optical throughput of each telescope, leading to both inter-telescope calibration and an absolute calibration of the entire observatory. One important aspect of the onsite calibration is a correct understanding of the atmosphere above the telescopes, which constitutes the calorimeter of this detection technique. It is planned to be constantly monitored with state-of-the-art instruments to obtain a full molecular and aerosol profile up to the stratosphere. In order to guarantee the best use of the observation time, in terms of usable data, an intelligent scheduling system is required, which gives preference to those sources and observation programs that can cope with the given atmospheric conditions, especially if the sky is partially covered by clouds, or slightly contaminated by dust. Ceilometers in combination with all-sky-cameras are plannned to provide the observatory with a fast, online and full-sky knowledge of the expected conditions for each pointing direction. For a precise characterization of the adopted observing direction, wide-field optical telescopes and Raman Lidars are planned to provide information about the height-resolved and wavelength-dependent atmospheric extinction, throughout the field-of-view of the cameras

Using muon rings for the optical calibration of the ASTRI telescopes for the Cherenkov Telescope Array

High-energy muons constitute a very useful tool to calibrate the total optical throughput of any telescope of the Cherenkov Telescope Array (CTA). Differences in precision and efficiency can however be present due to the variety of telescope types and sizes. In this contribution we present some preliminary results on simulated muon ring images collected by the ASTRI small sized dual-mirror (SST-2M) telescope in the basic configuration installed in Italy at the Serra La Nave observing station. ASTRI SST-2M is able, using 6% of the detected muon events, to calibrate with muons the optical throughput down to a degradation of the optical efficiency of 30%. Moreover, its precision in reconstructing the muon arrival direction is about one camera pixel, and its error on the reconstructed ring radius is 6.3%. The adopted procedures will be tested and validated with real data acquired by the prototype after the commissioning phase. The nine telescopes that will form the ASTRI mini-array, proposed to be installed at the final CTA southern site during the pre-production phase, will improve these results thanks to the higher detection efficiency and the lower optical cross-talk and after-pulse of their updated silicon photomultipliers. <P /