The ASTRI mini-array within the future Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) is a large collaborative effort aimed at the design and operation of an observatory dedicated to very high-energy gamma-ray astrophysics in the energy range from a few tens of GeV to above 100 TeV, which will yield about an order of magnitude improvement in sensitivity with respect to the current major arrays (H.E.S.S., MAGIC, and VERITAS). Within this framework, the Italian National Institute for Astrophysics is leading the ASTRI project, whose main goals are the design and installation on Mt. Etna (Sicily) of an end-to-end dual-mirror prototype of the CTA small size telescope (SST) and the installation at the CTA Southern site of a dual-mirror SST mini-array composed of nine units with a relative distance of about 300 m. The innovative dual-mirror Schwarzschild-Couder optical solution adopted for the ASTRI Project allows us to substantially reduce the telescope plate-scale and, therefore, to adopt silicon photo-multipliers as light detectors. The ASTRI mini-array is a wider international effort. The mini-array, sensitive in the energy range 1-100 TeV and beyond with an angular resolution of a few arcmin and an energy resolution of about 10-15%, is well suited to study relatively bright sources (a few $\times 10^{-12}$erg cm$^{-2}$s$^{-1}$ at 10 TeV) at very high energy. Prominent sources such as extreme blazars, nearby well-known BL Lac objects, Galactic pulsar wind nebulae, supernovae remnants, micro-quasars, and the Galactic Center can be observed in a previously unexplored energy range. The ASTRI mini-array will extend the current IACTs sensitivity well above a few tens of TeV and, at the same time, will allow us to compare our results on a few selected targets with those of current (HAWC) and future high-altitude extensive air-shower detectors.Comment: Proceedings of the "The Roma International Conference on Astroparticle Physics (RICAP) 2014". Submitted to EPJ Web of Conferences. 5 pages, 5 figures. ((1) INAF/IASF Palermo, (2) http://www.brera.inaf.it/astri/, (3) https://portal.cta-observatory.org

AGILE and blazars: the unexpected, the unprecedented, and the uncut

AGILE has been coordinating multi-wavelength campaigns on active galactic nuclei with several major observing facilities since its launch in 2007. This effort allowed us to investigate some remarkable sources both on short and on long time-scales, deriving information on the physical mechanisms responsible for the emission in different energy bands. A complete review of the whole set of AGILE results on extra-galactic sources and their theoretical interpretation is well beyond the scope of this paper, therefore I will present an overview on an handful of outstanding objects and describe the most recent observations

The mini-array of ASTRI SST-2M telescopes, precursors for the Cherenkov Telescope Array

In the framework of the Cherenkov Telescope Array (CTA) Observatory, the Italian National Institute of Astrophysics (INAF) has recently inaugurated in Sicily (Italy), at the Serra La Nave astronomical site on the slopes of Mount Etna, a large field of view (FoV, ∼ 9.6°) dual-mirror prototype (ASTRI SST-2M) of the CTA small size class of telescopes (SST). The CTA plans to install about 70 SST in the southern site to allow the study of the gamma rays from a few TeV up to hundreds of TeV. The ASTRI SST-2M telescope prototype has been developed following an end-to-end approach, since it includes the entire system of structure, mirrors optics (primary and secondary mirrors), camera, and control/acquisition software. A remarkable performance improvement could come from the operation of the ASTRI mini-array, led by INAF in synergy with the Universidade de Sao Paulo (Brazil) and the North-West University (South Africa). The ASTRI mini-array will be composed of nine ASTRI SST-2M units and it is proposed as a precursor and initial seed of the CTA to be installed at the final CTA southern site. Apart from the assessment of a number of technological aspects related to the CTA, the ASTRI mini-array will, if compared for instance to H.E.S.S., extend the point source sensitivity up to ∼ 100 TeV, also improving it above 5-10 TeV. Moreover, the unprecedented width of the FoV, with its homogeneous acceptance and angular resolution, will significantly contribute to the achievement of original results during the early CTA science phase

XMM-Newton observation of a spectral state transition in the peculiar radio/X-ray/gamma-ray source LS I +61 303

We report the results of XMM-Newton and BeppoSAX observations of the radio and X-ray emitting star LS I +61 303, likely associated with the gamma-ray source 2CG 135+01 and recently detected also at TeV energies. The data include a long XMM-Newton pointing carried out in January 2005, which provides the deepest look ever obtained for this object in the 0.3-12 keV range. During this observation the source flux decreased from a high level of 13E12 erg/cm2/s to 4E12 erg/cm2/s within 2-3 hours.This flux range is the same seen in shorter and less sensitive observations carried out in the past, but the new data show for the first time that transitions between the two levels can occur on short time scales. The flux decrease was accompanied by a significant softening of the spectrum, which is well described by a power law with photon index changing from 1.62+/-0.1 to 1.83+/-0.1. A correlation between hardness and intensity is also found when comparing different short observations spanning almost 10 years and covering various orbital phases.LS I +61 303 was detected in the 15-70 keV range with the PDS instrument in one of the BeppoSAX observations, providing evidence for variability also in the hard X-ray range. The X-ray spectra, discussed in the context of multiwavelength observations, place some interesting constraints on the properties and location of the high-energy emitting region.Comment: Revised version, accepted for publication in A&A. Updated references, few typos corrected, minor changes following referee's suggestion

The key science projects of the Cherenkov telescope array

The Cherenkov Telescope Array (CTA) will be the next generation gamma-ray observatory, open to the scientific community, to investigate the very high-energy emission from a large variety of celestial sources in the energy range 20 GeV - 300 TeV. The full array, distributed over two sites, one in the northern and one in the southern hemisphere, will provide whole-sky coverage and will improve by about one order of magnitude the sensitivity with respect to the current major arrays. CTA will investigate a much higher number of already known classes of sources, going to much larger distances in the Universe, performing population studies, accurate variability and spatially-resolved studies. Moreover, new light will be shed on possible new classes of high-energy sources, such as GRBs, cluster of Galaxies, Galactic binaries, and on fundamental physics. By pushing the high-energy limit to E > 100 TeV it will allow a thorough exploration of the cut-off regime of the cosmic accelerators. We review the main CTA Key Science Projects, which will focus on major scientific cases, a clear advance beyond the current state of the art, and we discuss the production of legacy data-sets of high value to a wider community

Perspectives of blazar studies with future space missions

Since the AGILE and Fermi launch, the synergy between gamma-ray experiments and other space- and ground-based observatories has been the key to carry out multi-wavelength campaign aimed at understanding the physical mechanisms responsible for the observed gamma-ray emission in astrophysical sources. Blazars are the best examples of astrophysical sources where this strategy has been applied. The big efforts put in place for blazars to obtain coordinated observations with a broad coverage of the electromagnetic spectrum are providing new diagnostics of the physical processes at work in these sources, raising a lot of challenges for the theoretical interpretation. These could be partially solved through further observations with ground- and space-based facilities, therefore requiring new advances in technology and mission profile design. We will discuss how the lessons learned from current -ray observatories represent an important heritage for future missions expected to play a crucial role in the understanding of extreme phenomena in the high-energy domain

The ASTRI Project in the Framework of the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) will be the next generation facility to investigate the very high-energy gamma-ray emission from a large variety of celestial sources. The full array, installed at two sites, one in the northern and one in the southern hemisphere, will start to operate at the beginning of the next decade. In the meantime, a few telescope prototypes have been developed and some pre-production CTA telescopes have been planned. Within this framework, the Italian National Institute for Astrophysics (INAF) is leading the ASTRI project, whose aim is two-fold. The ASTRI Collaboration has successfully developed and installed in Sicily a prototype of the CTA small-sized telescopes (SST), according to an innovative dual-mirror (2M) optical solution and equipped with a silicon-based photo-detector Cherenkov camera. Moreover, INAF is leading the development of one of the mini-arrays of pre-production CTA telescopes composed of at least nine dual-mirror telescopes and proposed to be installed at the CTA southern site in 2018. The ASTRI mini-array of SST pre-production CTA telescopes will be able to both verify some of the adopted innovative solutions, such as the wide field of view, and to investigate sources emitting at energies from a few TeV up to hundreds of TeV. We discuss the preliminary results obtained by the ASTRI SST-2M prototype during its ongoing commissioning phase, the expected Monte Carlo performance of the ASTRI mini-array of SST pre-production CTA telescopes, and provide an overview of the scientific topics that can be addressed both, as a stand-alone mini-array and in synergy with other pre-production CTA telescopes

Putting the hadron beam scenario for extreme blazars to the test with the Cherenkov Telescope Array

Hadron beams are invoked to explain the peculiar properties of a subclass of BL Lac objects, the so-called extreme BL Lacs (EHBLs). This scenario predicts a quite distinctive feature for the high-energy gamma-ray spectrum of these sources, namely a hard energy tail extending up to ∼100 TeV. It has been proposed that the detection of this tail can offer an unambiguous way to distinguish between the hadron beam scenario and the standard one, which instead assumes gamma-ray emission from the jet strongly depleted at the highest energies (E > 30 TeV) because of the interaction with the optical-IR cosmic radiation field. We present dedicated simulations of observations through the presently under construction Cherenkov Telescope Array (CTA) of the very high energy spectrum of the prototypical EHBL 1ES 0229+200 assuming the two alternative models. We demonstrate that, considering 50 h of observations from the southern site of CTA (the most sensitive at the highest energies), in the case of the hadron beam model it is possible to detect the source up to 100 TeV. This, together with the non-detection of the source above 10 TeV in the standard case, ensures that CTA observations can be effectively used to unambiguously confirm or rule out the hadron beam scenario