Telescopes and Instrumentation DOI: 10.18727/0722-6691/5021 The Cherenkov Telescope Array: Exploring the Very-high-energy Sky from ESO’s Paranal Site Werner Hofmann1 for the CTA Consortium Vulcano Llullaillaco 6739 m, 190 km east ESO/M. Tarenghi 1 Max-Planck-Institut für Kernphysik, Heidelberg, Germany Cerro Armazones ELT The Cherenkov Telescope Array (CTA) is a next-generation observatory for ground-based very-high-energy gamma- ray astronomy, using the imaging atmos- Cherenkov Telescope Array Site pheric Cherenkov technique to detect and reconstruct gamma-ray induced air showers. The CTA project is planning to deploy 19 telescopes on its northern Cerro Paranal La Palma site, and 99 telescopes on its southern site at Paranal, covering VISTA Very Large the 20 GeV to 300 TeV energy domain Telescope and offering vastly improved perfor- mance compared to currently operating Cherenkov telescopes. The combination of three different telescope sizes (23-, 12- and 4-metre) allows cost-effective coverage of the wide energy range. based detectors that are limited to detec- Figure 1. The location of the CTA southern array, CTA will be operated as a user facility, tion areas of around a square metre, in the valley between Cerro Paranal and Cerro Armazones. dividing observation time between Cherenkov telescopes use the Earth’s a guest observer programme and large atmosphere as a detection medium and Figure 2. Detection of primary γ-rays using the Key Science Projects (KSPs), and the provide detection areas in excess of Cherenkov light from the γ-ray induced air showers. data will be made public after a one- 105 square metres, capable of coping A telescope will “see” the γ-ray if it is located in the year proprietary period. The history of with the very low flux of VHEγ -rays. ~ 250-metre diameter Cherenkov light pool. The the project, the implementation of the Today’s instruments use arrays of IACTs different views of the air shower provided by multiple telescopes allow reconstruction of the shower arrays, and some of the major science to image a cascade from different view- geometry and hence of the direction of the incident goals and KSPs, are briefly summarised. ing angles, improving angular and energy γ-ray. Introduction Primary γ γ-ray enters the atmosphere In the coming years, up to 99 of the + e – Cherenkov Telescope Array (CTA) tele- Electromagnetic cascade e scopes with dish sizes between 4 metres + e γ γ and 23 metres will be installed at the e– + e – Paranal site, in the flat areas east of route e e+ γ e+ γ e– B-710 (Figure 1). CTA will be the premier e– observatory for imaging the Universe at very high energies (VHE), covering the electromagnetic spectrum at energies from 10’s of GeV to 100’s of TeV. More information can be found in Acharya et al. (2013) and on the CTA website1. CTA employs Imaging Atmospheric Cherenkov Telescopes (IACTs) — large telescopes with ultraviolet–optical reflecting mirrors that focus flashes of 10 nanosecond snapshot Cherenkov light produced by γ-ray initi- ated atmospheric particle cascades (air 0.1 km 2 “light pool”, a few photons per m2 showers) onto nanosecond-response cameras (Figure 2). Compared to space- The Messenger 168 – June 2017 21 Telescopes and Instrumentation Hofmann W., The Cherenkov Telescope Array resolution of the γ-rays, as well as speed of light with photon energy; and 2015, the RB decided to enter into increasing the rejection of similar cas- photon-axion oscillations in cosmic detailed contract negotiations around cades initiated by cosmic-ray particles. magnetic fields. hosting the southern array on the Paranal site in Chile and the northern array at the Ground-based γ-ray astronomy at very CTA is envisaged as a general- purpose Instituto de Astrofisica de Canarias (IAC), high energies is a young branch of observatory for the VHE waveband, Roque de los Muchachos Observatory astronomy that has developed very rapidly building on the techniques and technolo- in La Palma, Spain. The hosting agree- since the detection of the first cosmic gies demonstrated by the currently oper- ment with IAC was signed in September VHE source in 1989 by the Whipple tele- ating IACTs, and improving on essentially 2016; the hosting agreement with ESO scope (Hillas, 2013). The initial concepts all aspects of their performance. CTA was approved in late 2016 by both the for CTA as the first major open obser- will be the first truly open VHE observa- CTAO and ESO Councils. It is envisaged vatory for this waveband were formulated tory, providing accessible data products that ESO will become a scientific partner in 2005, motivated by the success of and support services to a wide scientific in CTA, expanding ESO’s portfolio of existing IACTs, such as the High Energy community. It will exploit large arrays wavebands and leveraging the scientific Stereoscopic System (HESS), the Major of Cherenkov telescopes on two sites to and operational synergies with its optical Atmospheric Gamma Imaging Cherenkov provide all-sky coverage, broad energy telescopes and the Atacama Millimeter/ (MAGIC) and the Very Energetic Radia- coverage and unprecedented precision. submillimeter Array (ALMA). ESO will tion Imaging Telescope Array System operate the southern telescope array for (VERITAS). These instruments have dem- CTAO and will receive observation time onstrated that observations at these Some history on CTA’s arrays as well as voting rights in extreme energies are not only technically the CTAO’s governing bodies. Also in viable and competitive in terms of pre- The CTA project was proposed and 2016, the decision was taken to locate cision and depth, but also scientifically developed by the CTA Consortium the CTA Headquarters at Bologna in Italy; rewarding and with broad scientific (CTAC) that was formally established in the Science Data Management Centre impact. Their success has resulted in a 2008. The Consortium has now grown will be hosted at Zeuthen near Berlin, rapid growth in the interested scientific to over 1300 scientists and engineers Germany. community. Topics addressed with γ-ray from more than 200 institutes in 32 coun- observations include2: tries, involved in the design and proto- The full economic cost for implementing (i) The origin and role of relativistic cosmic typing of the telescopes and the associ- CTA is estimated at 400 M€; however, particles. Particles in our Galaxy and ated auxiliary instruments and software, even with a reduced number of tele- beyond are traced by the γ-rays they as well as in the characterisation of sites scopes, CTA will provide a state-of-the- emit when interacting with gas or with for the telescopes. Of ESO’s 16 Member art astronomical facility, and a funding radiation fields; this allows us to address States, 13 are also represented in the level of 250 M€ was established as the questions like: what are the sites of CTA Consortium. In 2014, the CTA Obser- threshold for starting the implemen tation. high-energy particle acceleration in the vatory (CTAO) gGmbH was founded in Signature of a Memorandum of Under- Universe? what are the mechanisms Heidelberg, to provide a legal framework standing (MoU) towards construction and for cosmic particle acceleration? and for the operation of the CTA Project operation is underway and currently what role do accelerated particles play Office, and for the contracts towards accounts for over 200 M€; the 250 M€ in feedback mechanisms related to star implementation of CTA. The CTAO gGmbH threshold should be reached in the near formation and galaxy evolution? is governed by its Council of representa- future. (ii) Probing extreme environments, for tives of the shareholders from Austria, the example, the physical processes that Czech Republic, France, Germany, Italy, are at work close to neutron stars Japan, Spain, Switzerland and the United The telescope arrays and black holes and the characteris- Kingdom, and from the Netherlands and tics of relativistic jets, winds and explo- South Africa as Associate Members. In all aspects, CTA represents a signifi- sions. γ-ray interactions can also be Work towards CTA was and is supported cant step forward with respect to current used to explore the radiation fields and by the European Union under FP7 and instruments, and the combined effect is magnetic fields in extreme cosmic H2020; since 2008, CTA is listed in the expected to be transformational for the voids, and their evolution over cosmic roadmap of the European Strategy Forum field. For example, the improvedγ -ray time. on Research Infrastructures (ESFRI). collection area, the background rejection (iii) Exploring frontiers in fundamental power and the larger field of view increase physics, such as: searching for dark A comprehensive programme of site the survey speed of CTA by a factor of matter particles annihilating into search and site evaluation was conducted several hundred with respect to current γ-rays, allowing us to probe the nature by the CTA Consortium from 2010 to instruments. Sensitivity to point sources and distribution of dark matter; inves- 2013, resulting in a shortlist of sites and of γ-rays will be up to an order of magni- tigating mechanisms affecting photon detailed input to a Site Selection Com- tude higher than current instruments. The propagation over cosmological dis- mittee appointed by the CTA Resource angular resolution of CTA will approach tance, such as quantum gravitational Board (RB) of agency representatives (the one arcminute at high energies — the effects causing tiny variations of the RB preceded the CTAO Council). In July best resolution of any instrument operat- 22 The Messenger 168 – June 2017 CTA/Gabriel Pérez Diaz/IAC/SMM/ESO/B.Tafreshi Pérez CTA/Gabriel Figure 3. (Above) Artist’s ing above the X-ray band — allowing Southern hemisphere detailed imaging of a large number of rendering of the south- Type: ern CTA site.
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