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 implemen­tation. 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. γ-ray sources. 23-M LST CTA will take the IACT technique to its 12-M MST 4-M SST next level, by deploying extended arrays of Cherenkov telescopes (Figures 3 and 4). In the current arrays, with at most five telescopes spaced by about 100 metres (compared to the ~ 250-metre diameter of the Cherenkov light pool), the bulk of the recorded air showers have impact points outside the footprint of the array, 1000 m implying that usually only two telescopes record the shower, and that the angle between stereoscopic views of the cas- Figure 4. (Right) Layout Circles: of the southern tele- – 400 m cade is modest, impacting the spatial scope array, combining – 800 m – 1200 m reconstruction. CTA will for the first time 4 LSTs, 25 MSTs and deploy telescopes across areas that 70 SSTs. 4 LSTs, 25 MSTs, 70 SSTs exceed the size of the Cherenkov light pool, resulting in: area to increase with γ-ray energy, com- − The highest energy γ-rays are detected − a dramatically increased rate of air pensating for the rapid drop of γ-ray flux by a multi-kilometre square array of 70 showers contained within the footprint with increasing energy (for typical sources, Small-Sized Telescopes (4-metre SSTs) of the telescope array; the γ-ray flux drops with energy, E, like in the south. −2 − an increased number of views of the air dNγ/dE ~ E or faster). Hence, rather than shower from different viewing angles, deploying one type of Cherenkov tele- The small telescopes are only foreseen improving both the reconstruction of scope on a regular grid, the CTA arrays for the southern array (Figure 4), since the air-shower parameters and the rejection use a graded approach: highest energies are most relevant for of cosmic-ray induced air showers as − The lowest energies are covered by the study of Galactic sources. The use of the major source of sensitivity-limiting an arrangement of four Large-Sized three different sizes of telescope proved background; Telescopes (LSTs, of 23-metre dish to be the most cost-effective solution, − a lower effective energy threshold diameter), capable of detecting γ-rays and it allows each telescope type to be since, for contained showers, there are as low as 20 GeV. optimised for a specific energy range. always telescopes in the region of high- − The 0.1 to 10 TeV range is covered by The detailed specifications of the tele- est density of Cherenkov light. larger arrays of 25 (south) and 15 (north) scopes and the layout of the CTA arrays Medium-Sized Telescopes (12-metre are the result of a multi-step optimisation For wide usable energy coverage, it is MSTs). process, extending over several years desirable for the effective γ-ray detection and using 10’s of millions of CPU-hours

The Messenger 168 – June 2017 23 Telescopes and Instrumentation Hofmann W., The Cherenkov Telescope Array

Telescope Large Medium Small LST MST SCT SST-1M ASTRI SST-2M GCT SST-2M Number North array 4 15 TBD 0 Number South array 4 25 TBD 70 Optics Optics Parabolic mirror Modified Davies-­ Schwarzschild- Davies-Cotton Schwarzschild- Schwarzschild- Cotton Couder Couder Couder Primary mirror diameter (m) 23 12 9.7 4 4 4 Secondary mirror diameter (m) — — 5.4 — 1.8 2 Eff. mirror area after shadowing (m2) 370 90 40 7.4 6 6 Focal length (m) 28 16 5.6 5.6 2.1 2.3 Focal plane instrumentation Photo sensor PMT PMT Silicon Silicon Silicon Silicon Pixel size (degr.), shape 0.10, hex 0.18, hex 0.07, square 0.24, hex 0.17, square 0.15–0.2, square Field of view (degr.) 4.5 7.7/8.0 8.0 9.1 9.6 8.5–9.2 Number of pixels 1855 1764/1855 11328 1296 1984 2048 Signal sampling rate GHz 250 MHz/GHz GHz 250 MHz S&H GHz Structure Mount alz-az, on circular rail alt-az positioner alt-az positioner alt-az positioner alt-az positioner alt-az positioner Structural material CFRP/steel steel steel steel steel steel Weight (full telescope, tons) 100 85 ~85 9 15 8 Max. time for repositioning (s) 20 90 90 60 80 60

Table 1. Parameters of the different CTA telescopes currently under prototyping. For details of telescope technology, see Gamma, 20163. GCT MSTSCT

for detailed simulations of the air showers and the response of the telescopes

Gabriel Pérez Diaz, IAC, SMM (Bernlöhr et al., 2013).

The parameters of the telescopes are SST-1M summarised in Table 1. The different tele- scopes (Figure 5) have rather different design drivers. For example, the main drivers for the LSTs are the huge light- collecting power and rapid repositioning requirements (needed in particular for ­follow-up of γ-ray bursts), and for the SSTs ASTRI the large number of elements implies tight constraints on unit cost and mainte- nance effort. Three different SST designs are being prototyped, the Gamma-ray

Figure 5. Main image: CAD models of the different CTA telescopes, shown to scale. The Small-Sized Telescopes (SST, from left: GCT, ASTRI, SST-1M) have a mirror diameter of about 4 metres, the Medium-Sized Telescope (MST, middle) has a 12-metre mirror, and the Large-Size Telescope (LST) a 23-metre mirror. Insets: Prototypes of CTA tele- scopes under test: GCT (at Meudon), ASTRI (on ­Sicily), SST-1M (at Warsaw),­ MST (at Berlin), and SCT (in Arizona). ­Construction of an LST prototype has started on La Palma (not shown).

24 The Messenger 168 – June 2017 Figure 6. Simulation of the Milky Way seen with CTA ) 4

in very-high-energy γ-rays. eg 2 (d 0 de Cherenkov Telescope (GCT) and the titu –2

Astrofisica con Specchi a Tecnologia La –4 Replicante Italiana (ASTRI) dual-mirror 90 80 70 60 50 40 30 telescopes and the SST-1M single-mirror

) 4

variant. The dual-mirror telescopes use eg 2 Schwarzschild-Couder (SC) optics, real- (d 0 ised for the first time in CTA’sCherenkov ­ de

titu –2 telescopes. Also, a dual-mirror version La –4 of the MST is being considered (SCT); compared to the classical single-mirror 30 20 10 0 –10–20 –30

Cherenkov telescopes; the novel dual- ) 4 mirror telescopes offer improved imaging eg 2 (d 0 quality and allow a smaller plate scale de and rather compact cameras. The LST titu –2 and MST with their large (3-metre) La –4 ­cameras use photomultipliers with very 330 320310 300 290280 270 high quantum efficiency to detect the Longitude (deg) Cherenkov light, whereas the smaller sensor areas of the SSTs and the SCT ­telescope configurations, and specify lactic Survey; (vi) Transients; (vii) Cosmic- are covered with silicon sensors. trigger conditions, time constraints aris- ray PeVatrons; (viii) Star-forming Systems; ing from coordinated observations with (ix) Active Galactic Nuclei; (x) Clusters of other observatories, etc. Users will Galaxies; and (xi) Beyond gamma-rays. The CTA Observatory receive processed and calibrated data in In the following, only the survey KSPs are the form of photon-candidate event lists briefly introduced; these will form a basis The CTA facility will be open to a wide similar to the products of other photon- for subsequent guest observer proposals. community of scientific users: from counting instruments, provided in the astronomy and astrophysics, to astropar­ Flexible Image Transport System (FITS) The combination of CTA’s wide field of ticle physics, particle physics, cosmology format, together with relevant instrument view (FoV) and its unprecedented sensi- and plasma physics. CTA observation response functions and auxiliary data, tivity ensures that CTA can deliver sur- time and data products will be provided as well as science analysis tools. veys 1–2 orders of magnitude deeper to users through several different modes: than existing surveys very early in the life – The Guest Observer Programme is Science data, calibration data and auxil- of the CTA Observatory. CTA surveys will a proposal-based system that will give iary data will be permanently stored in the open up discovery space in an unbiased users access to time for observations CTA data archive and made available to way and generate legacy datasets of requiring anywhere between a few hours archive users after the proprietary period. long-lasting value. The following surveys and hundreds of hours. The data archive will remain available form part of the KSPs: – The Key Science Projects (KSPs) beyond the operational period of the CTA − The Extragalactic Survey — covering defined by the CTA Consortium are Observatory. High-level data, such as one quarter of the sky to a depth of large programmes that ensure that the sky maps, light curves, and source cata- ~ 6 mCrab. No extragalactic survey has key science issues are addressed in a logues will be made available in a Virtual ever been performed using IACTs. A coherent fashion, generating legacy Observatory (VO) compliant format. 1000-hour CTA survey of such a region data products. KSPs require anything Alerts from CTA on noteworthy astro- will reach the same sensitivity as the between 100 and 1000 hours of obser- nomical events (for example the detection decade-long HESS programme of vation time. of transients) will be sent to the wider inner-Galaxy observations, and cover a – The Director’s Discretionary Time will community in the VO Event format. solid angle ~ 40 times larger, providing represent a small fraction of observa- a snapshot of activity in an unbiased tion time that will be used for, for exam- sample of galaxies. ple, unanticipated targets of opportu- The CTA Key Science Projects − The Galactic Plane Survey (GPS) — nity or outstanding proposals from consisting of a deep survey (~ 2 mCrab) non-member countries. The detailed volume Science with the of the inner Galaxy and the Cygnus – Archive Access to CTA data will be Cherenkov Telescope Array is about region, coupled with a somewhat shal- made available worldwide after a pro- to be published. Proposed CTA KSPs lower survey (~ 4 mCrab) of the entire prietary period of about one year. include: (i) the Dark Matter Programme; Galactic Plane (see Figure 6). For the (ii) the Galactic Centre Survey; (iii) the typical luminosity of known Milky Way Observations will take place in service Galactic Plane Survey; (iv) the Large TeV sources of 1033–34 erg s–1, the mode; the user may request specific Magellanic Cloud Survey; (v) the Extraga- CTA GPS will provide a distance reach

The Messenger 168 – June 2017 25 Telescopes and Instrumentation Hofmann W., The Cherenkov Telescope Array

of ~ 20 kpc, detecting essentially the surveys is not only that they provide the of the full scope of the arrays may pro- entire population of such objects in the basis for a population synthesis of cosmic gress gradually. Science operation will Galaxy and providing a large sample particle accelerators, but also that they start with partial arrays, before deploy- of objects one order of magnitude enable discovery of key objects — the ment of the full arrays is completed. Tele- fainter. The excellent angular resolution equivalents of the Hulse-Taylor pulsar or scopes are successively handed over of CTA is critical here if it is to avoid the Double Pulsar found in radio pulsar from the construction project to opera- being limited by source confusion surveys. tions. Prior to user science operation, the rather than flux. performance of the (initially partial) arrays − The Galactic Centre Survey — con­ will be verified and documented. It is sisting of a > 500 hours deep exposure Construction of the CTA anticipated that construction activities will of a ± 1 degree window around Sgr A*, start in 2018, with the first data from par- covering the Galactic Centre source, The Design Phase of CTA has been con- tial arrays becoming available in 2021. the centre of the Dark Matter halo, cluded; the project is currently near the ­multiple supernova remnants and pul- end of the Pre-Construction Phase in In summary, CTA will provide a next-­ sar wind nebulae, central radio lobes which instrument designs were evolved to generation VHE γ-ray observatory with and arc features. An additional 300 production readiness and were reviewed unprecedented capability. CTA perspec- hours extended survey covers a 10 by in the Science Performance and Require- tives in relativistic astrophysics include 10 degree region around Sgr A*, includ- ments Review, the Preliminary Design the in-depth understanding of known VHE ing the edge of the Galactic Bulge, Review and the final Critical Design γ-ray emitters and their mechanisms, the base of the Fermi Bubbles, the Review — all performed by CTA’s Sci- detection of new object classes and dis- radio spurs and the Kepler supernova ence and Technical Advisory Committee. covery of new phenomena. As with many remnant. facilities breaking new ground, the most − The Large Magellanic Cloud (LMC) The forthcoming Pre-Production Phase important discoveries may not be the ­Survey — providing a face-on view covers the deployment of approximately ones discussed in today’s science case of an entire star-forming galaxy, resolv- 10 % of the final number of telescopes documents. ing regions down to 20 pc in size with with the aim of verifying and optimising sensitivity down to luminosities of production schemes, as well as exer­cising ~1034 erg s–1. CTA aims to map the dif- and optimising deployment, testing soft- References fuse LMC emission as well as individual ware, etc. Based on the Pre-Production Acharya, B. S. et al. 2013, Astroparticle Physics, objects, providing information on rela- Readiness/Deployment Reviews, Pre- 43, 3 tivistic particle transport. Production telescopes will be installed Bernlöhr, K. et al. 2013, Astroparticle Physics, on the sites; after retrofitting­ to final pro- 43, 171 These surveys will establish the popula- duction status where ­relevant, they are Hillas, A. M. 2013, Astroparticle Physics, 43, 19 tions of VHE emitters in Galactic and planned to be used in the final CTA arrays. extragalactic space, providing samples Links of objects large enough to understand Once Pre-Production is successfully 1 source evolution and/or duty cycle. Data established for a given telescope type, CTA Observatory: www.cta-observatory.org 2 For an overview of CTA science, see articles in the products from the survey KSPs include this element moves to the Production Special CTA edition of Astroparticle Physics, 43, catalogues and flux maps which will Phase. To first approximation, telescopes 1-356, 2013 serve as valuable long-term resources are not dependent on each other for 3 Contributions of CTA to the 6th International for a wide community. The value of these operation and the cumulative completion ­Symposium on High-Energy Gamma-Ray ­Astronomy (Gamma 2016), arXiv:1610.05151

The panorama from the planned site of the Cherenkov Telescope Array looking towards the Very Large ­Telescope on Cerro Paranal.

26 The Messenger 168 – June 2017