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ASTRI ("Astrofisica con Specchi a Tecnologia Replicante Italiana")

Group members: Patrizia Caraveo, Andrea De Luca, Mauro Fiorini, Salvatore Incorvaia, Nicola La Palombara, Nicola Sartore, Luca Stringhetti

ASTRI ("Astrofisica con Specchi a Tecnologia Replicante Italiana") is a flagship project of the Italian Ministry of Education, University and Research strictly linked to the development of the ambitious Cherenkov Telescope Array (CTA).

CTA plans the construction of many tens of telescopes divided in 3 kinds of configurations, in order to cover the energy range from a tens of GeV (Large Size Telescope, LST), to a tens of TeV (Medium Size Telescope, MST), and up to 100 TeV (Small Size Telescope, SST). Within this framework, INAF is currently developing an end-to-end prototype of the CTA small-size telescope in a dual-mirror configuration (SST-2M) to be tested under field conditions, and scheduled to start data acquisition in 2014.

INAF is in charge of the design of the mirrors and the focal plane, development of the Monte-Carlo, control, acquisition, data-handling and archiving software and end-to-end operations, while the telescope structure is designed by external firms. Moreover, INAF supports all activities related to the finalization of the CTA technological aspects, such the participation to the CTA management and Project Office activities, the Monte-Carlo simulations, and the data handling.

For the first time, a wide field of view (FoV = 9.6 degrees in diameter) dual-mirror Schwarzschild-Couder (SC) optical design will be adopted on a Cherenkov telescope, in order to obtain a compact (F# = 0.5) optical configuration and equipped with a light (~50 kg) and compact (~50cm×50cm×50cm) camera based on Hamamatsu S11828-3344M Silicon photo-multipliers which offer photon detection sensitivity in the 300-700nm band, a fast temporal response and a suitable logical pixel size of 6.2mm×6.2mm. The proposed Telescope layout (whose mount exploits the classical alt-azimuthal configuration) is fully compliant with the CTA requirements for the SST array. The telescope design is compact having a 4.3m-diameter segmented primary mirror, a 1.8m diameter monolithic secondary mirror, and a primary-to-secondary distance of 3m. The SC optical design has a focal ratio F# = 0.5, a plate scale of 37.5mm/degree, a logical pixel size of approximately 0.17 degrees and an equivalent focal length of 2150mm. Considering 1984 pixels, this setup delivers the above mentioned FoV and a mean value of the active area of about 6.5 square meters, taking into account all the possible attenuation and occultation factors.

The ASTRI Prototype will be placed at Serra La Nave, 1735m a.s.l. on the Etna Mountain near Catania, at the INAF "M.G. Fracastoro" observing station. Although the ASTRI SST-2M prototype is mainly a technological demonstrator, it will perform scientific observations on the Crab Nebula, Mrk 421 and Mrk 501. Preliminary calculations show that in the maximum sensitivity range (E>1 TeV) we can detect a flux level of 1 Crab at 5σ in a few hours, while in the energy range E>10 TeV the same flux at 5σ can be reached in a few tens of hours. A remarkable improvement in terms of performance could come from the operation, in 2016, of a SST-2M mini-array, composed by a few SST-2M telescopes and to be placed at final CTA Southern Site. The SST mini-array will be able to study in great detail relatively bright sources (a few 10-12 erg/cm2/s at 10 TeV) with an angular resolution of a few arcmin and an energy resolution of about 10-15%. Moreover, thanks to the array approach, it will be possible to verify the wide FoV performance to detect very high energy showers with the core located at a distance up to 500 m, to compare the mini-array performance with the Monte Carlo expectations (by means of well chosen target objects and long observations), and to perform the first CTA science (thanks to, e.g., about 5 solid detections during the first year of operation). Prominent sources such as PKS 2155-304, 1ES 0229+200, Mrk 421, M 87, Crab Nebula, Vela-X, Vela Junior, RX J1713.7-3946, HESS J1718-385, LS 5039, HESS J1825-137, and the Galactic Centre can be detected in order to investigate the electron acceleration and cooling, relativistic and non relativistic shocks, the search for cosmic-ray (CR) Pevatrons, the study of the CR propagation, and the impact of the extragalactic background light on the spectra of the sources.