5.3. CTA Performance Summary
Section 7 gives a detailed description of the layout and performance studies conducted so far for CTA. Many candidate layouts have been considered. Here we provide a brief description of the nature and performance of one promising configuration (E), which is illustrated in fig. 18. This configuration utilises three telescope types: four 24 m telescopes with 5◦ field-of-view and 0.09◦ pixels, 23 telescopes of 12 m diameter with 8◦ field-of-view
and 0.18◦pixels, and 32 telescopes of 7 m diameter with a 10◦field-of-view and 0.25◦pixels.
The telescopes are distributed over ∼3 km2 on the ground and the effective collection area
of the array is considerably larger than this at energies beyond 10 TeV. The sensitivity of array E from detailed calculations and using standard data analysis techniques is shown in fig. 23. More sophisticated analyses result in sensitivities that are ∼20% better across the whole energy range. As fig. 23 shows, such an array performs an order of magnitude better than an instrument like H.E.S.S. over most of required energy range. Fig. 25 shows the angular resolution of this array, which approaches one arcminute at high energies. The energy resolution of layout E is better than 10% above a few hundred GeV.
Array layout E has a nominal construction cost of 80 Me and meets the main design goals of CTA. Given that the configuration itself, and the analysis methods used, have not yet been optimised, it is likely that a significantly better sensitivity can be achieved with an array of this nominal cost which follows the same basic concept. Therefore, despite the uncertainties in the cost model employed (see sec. 6.5), we are confident that the design goals of CTA can be realised at close to the envisaged cost.
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6. Realizing CTA
This section provides a brief overview of the position of CTA in the European and global context, the organisation of CTA during the various stages, of its operation as an open observatory, of the potential sites envisaged for CTA, and of the schedule for and cost of CTA design, construction and operation.
6.1. CTA and the European Strategy in Astrophysics and Astroparticle Physics
CTA, as a major future facility for astroparticle physics, is firmly embedded in the European processes guiding science in the fields of astronomy and astroparticle physics. The European Strategy Forum on Research Infrastructures (ESFRI): ESFRI is a strategic organisation whose objective is to promote the scientific integration of Europe, to strengthen the European Research Area and to increase its international impact. A first Roadmap for pan-European research infrastructures was released in 2006, listing CTA as an “emerging project”. In the December 2008 update of this Roadmap, CTA was included as one of eight Physical Sciences and Engineering projects, together with facilities such as E-ELT, KM3Net and SKA. As such, CTA is eligible for FP7 Preparatory Phase funding. The CTA application for this funding was successful, providing up to 5.2 Me for the preparation of the construction of the observatory in 3 years time. The contracts with the EC are in the process of being finalised and signed.
The Astroparticle Physics European Coordination (ApPEC) group: ApPEC was created to enhance coordination in astroparticle physics across Europe. It has stimulated cooperation and convergence between competing groups in Europe, and has initiated the production of a European roadmap in astroparticle physics, on which CTA is one of the key projects.
ASPERA: ASPERA is a network of national government agencies responsible for coor- dinating and funding national research efforts in Astroparticle Physics. One of the tasks of ASPERA is to create a scientific roadmap for Astroparticle Physics [79] and link it with the more general European scientific infrastructure roadmap. A Phase I roadmap has been published, presenting the overarching science questions and the new instruments planned to address these questions. Phase II saw the release of the resulting “European Strategy for Astroparticle Physics” in September 2008, prioritising the projects under consideration. In this roadmap, CTA emerges as a near-term high-priority project. The roadmap states:
The priority project for VHE gamma-ray astrophysics is the Cherenkov Telescope Array, CTA. We recommend design and prototyping of CTA, the selection of sites, and proceeding rapidly towards start of deployment in 2012.
CTA was one of the two projects targeted by the 2009 ASPERA Common Call for cross-national funding and received in total 2.7 Me from national funding agencies. The ASTRONET Eranet: ASTRONET was created by a group of European fund- ing agencies to establish comprehensive long-term planning for the development of
6.2 CTA in the World-Wide Context 42
European astronomy. The objective of this effort is to consolidate and reinforce the world-leading position that European astronomy attained at the beginning of this century. Late in 2008, ASTRONET released “The ASTRONET Infrastructure Roadmap: A Strategic Plan for European Astronomy”. CTA is one of the three medium-scale facilities recommended on this roadmap, together with the neutrino telescope KM3Net and the solar telescope EST.
6.2. CTA in the World-Wide Context
Ground-based gamma-ray astronomy has attracted considerable attention world-wide, and while CTA is the key project in Europe, other projects have been considered elsewhere. These include primarily:
The Advanced Gamma-ray Imaging System (AGIS): In both science an instru- mentation, AGIS [80] followed a very similar plan to that of CTA. The AGIS project was presented in a White Paper prepared for the Division of Astrophysics of the American Physical Society [12]. AGIS proposed a square-kilometre array of mid- sized telescopes, similar to the core array of mid-sized telescopes in CTA but without the additional large telescopes to cover the very lowest energies, and an extended array of small telescopes to provide large detection area at the very highest ener- gies. The baseline configuration of AGIS consisted of 36 two-mirror Schwarzschild- Couder telescopes with an 11.5 m diameter primary mirror. These have a large field of view and a very good angular resolution. Close contacts were established between AGIS and CTA, during the design study phase; information was openly exchanged and common developments undertaken. After a US review panel recom- mended that AGIS join forces with CTA, the US members of the AGIS Collaboration have joined CTA in spring 2010. Within the overall context of CTA, development of Schwarzschild-Couder telescopes will be continued to investigate their potential for further improving CTA performance. Significant intellectual, technological and financial contributions to CTA from the US groups are anticipated. Strong US par- ticipation in CTA was endorsed by PASAG6 and the Decadal Survey in Astronomy
and Astrophysics (Astro-2010).
The High-Altitude Water-Cherenkov Experiment (HAWC): HAWC [81] builds on the technique developed by the MILAGRO group, which detects shower parti- cles on the ground using water Cherenkov detectors, and reconstructs the shower direction using timing information. It is proposed to construct the new detector on a site at 4100 m a.s.l. in the Sierra Negra, Mexico. HAWC will provide a tenfold increase in sensitivity over MILAGRO and detection capability down to the lower energy of 100 GeV, largely due to its increased altitude. While it will have lower sensitivity, poorer angular resolution and a higher energy threshold compared to CTA, HAWC has the advantage of a large field of view (≈ 2π sr) and nearly 100% duty cycle. HAWC therefore complements imaging Cherenkov instruments. In fact,
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