CTA (Cherenkov Telescope Array) Observatory in Chile and on the Island of La Palma, Spain
The Cherenkov Telescope Array, CTA, will be the major global observatory for very high energy gamma-ray astronomy over the next decade and beyond. The scientific potential of CTA is extremely broad: from understanding the role of relativistic cosmic particles to the search for dark matter. CTA is an explorer of the extreme universe, probing environments from the immediate neighborhood of black holes to cosmic voids on the largest scales. Covering a huge range in photon energy from 20 GeV to 300 TeV, CTA will improve on all aspects of performance with respect to current instruments. Wider field of view and improved sensitivity will enable CTA to survey hundreds of times faster than previous TeV telescopes. The angular resolution of CTA will approach 1 arc-minute at high energies — the best resolution of any instrument operating above the X-ray band — allowing detailed imaging of a large number of gamma ray sources. A one to two order-of-magnitude collection area improvement makes CTA a powerful instrument for time-domain astrophysics, three orders of magnitude more sensitive on hour timescales than Fermi-LAT at 30 GeV. The observatory will operate arrays on sites in both hemispheres to provide full sky coverage and will hence maximize the potential for the rarest phenomena such as very nearby supernovae, gamma-ray bursts or gravitational wave transients. With 99 telescopes on the southern site and 19 telescopes on the northern site, flexible operation will be possible, with sub-arrays available for specific tasks. 1)
CTA will have important synergies with many of the new generation of major astronomical and astroparticle observatories. Multiwavelength and multi-messenger approaches combining CTA data with those from other instruments will lead to a deeper understanding of the broad-band non-thermal properties of target sources, elucidating the nature, environment, and distance of gamma-ray emitters. Details of synergies in each waveband are presented.
The CTA Observatory will be operated as an open, proposal-driven observatory, with all data available on a public archive after a pre-defined proprietary period (of typically one year). Scientists from institutions worldwide have combined together to form the CTA Consortium. This Consortium has prepared a proposal for a Core Program of highly motivated observations. The program, encompassing approximately 40% of the available observing time over the first ten years of CTA operation, is made up of individual Key Science Projects (KSPs), which are presented in the subsequent chapters. The science cases have been prepared over several years by the CTA Consortium, with community input gathered via a series of workshops connecting CTA to neighboring communities. A major element of the program is the search for dark matter via the annihilation signature of weakly interacting massive particles (WIMPs). The strategy for dark matter detection presented here places the expected cross-section for a thermal relic within reach of CTA for a wide range of WIMP masses from ~200 GeV to 20 TeV. This makes CTA extremely complementary to other approaches, such as high-energy particle collider and direct-detection experiments. CTA will also conduct a census of particle acceleration over a wide range of astrophysical objects, with quarter-sky extragalactic, full-plane Galactic and Large Magellanic Cloud surveys planned. Additional KSPs are focused on transients, acceleration up to PeV (1015 eV) energies in our own Galaxy, active galactic nuclei, star-forming systems on a wide range of scales, and the Perseus cluster of galaxies. All provide high-level data products which will benefit a wide community, and together they will provide a long-lasting legacy for CTA.
Finally, while designed for the detection of gamma rays, CTA has considerable potential for a range of astrophysics and astroparticle physics based on charged cosmic-ray observations and the use of the CTA telescopes for optical measurements.
Introduction to CTA Science
Ground-based gamma-ray astronomy is a young field with enormous scientific potential. The possibility of astrophysical measurements at (TeV (1012 eV) energies was demonstrated in 1989 with the detection of a clear signal from the Crab nebula above 1 TeV with the Whipple 10 m IACT (Imaging Atmospheric Cherenkov Telescope). Since then, the instrumentation for, and techniques of, astronomy with IACTs have evolved to the extent that a flourishing new scientific discipline has been established, with the detection of more than 150 sources and a major impact in astrophysics and more widely in physics. The current major arrays of IACTs: H.E.S.S. (High Energy Stereoscopic System), MAGIC (Major Atmospheric Gamma Imaging Cherenkov Telescopes), and VERITAS (Very Energetic Radiation Imaging Telescope Array System), have demonstrated the huge physics potential at these energies as well as the maturity of the detection technique. Many astrophysical source classes have been established, some with many well-studied individual objects, but there are indications that the known sources represent the tip of the iceberg in terms of both individual objects and source classes. The Cherenkov Telescope Array (CTA) will transform our understanding of the high-energy universe and will explore questions in physics of fundamental importance. As a key member of the suite of new and upcoming major astroparticle physics experiments and observatories, CTA will exploit synergies with gravitational wave and neutrino observatories as well as with classical photon observatories. CTA will address a wide range of major questions in and beyond astrophysics, which can be grouped into three broad themes:
Theme 1: Understanding the Origin and Role of Relativistic Cosmic Particles
- What are the sites of high-energy particle acceleration in the universe?
- What are the mechanisms for cosmic particle acceleration?
- What role do accelerated particles play in feedback on star formation and galaxy evolution?
Theme 2: Probing Extreme Environments
- What physical processes are at work close to neutron stars and black holes?
- What are the characteristics of relativistic jets, winds and explosions?
- How intense are radiation fields and magnetic fields in cosmic voids, and how do these evolve over cosmic time?
Theme 3: Exploring Frontiers in Physics
- What is the nature of dark matter? How is it distributed?
- Are there quantum gravitational effects on photon propagation?
- Do axion-like particles exist?
Key Characteristics & Capabilities
CTA will be an observatory with arrays of IACTs (Imaging Atmospheric Cherenkov Telescopes) on two sites, aiming to:
• improve the sensitivity level of current instruments by an order of magnitude at 1 TeV
• significantly boost detection area, and hence photon rate, providing access to the shortest timescale phenomena,
• substantially improve angular resolution and field of view and hence ability to image extended sources,
• provide energy coverage for photons from 20 GeV to at least 300 TeV, to give CTA reach to high-redshifts and extreme accelerators,
• dramatically enhance surveying capability, monitoring capability, and flexibility of operation, allowing for simultaneous observations of objects in multiple fields,
• serve a wide user community, with provision of data products and tools suitable for non-expert users, and
• provide access to the entire sky, with sites in two hemispheres [the two sites are referred to as CTA-South (located at the Paranal-Armazones site, Chile, altitude of 2100 m) and CTA-North, located on the island of La Palma, Spain].
CTA will be operated as an open, proposal-driven observatory for the first time in very-high-energy (VHE, E>20 GeV) astronomy. The observatory-mode operation of CTA is expected to significantly boost scientific output by engaging a research community much wider than the historical ground-based gamma-ray astronomy community.
The very wide energy range covered by the southern CTA array necessitates the use of at least three different telescope types: referred to as Large, Medium and Small-Sized Telescopes (LSTs, MSTs and SSTs). The LSTs provide sensitivity at the lowest energies and SSTs at the highest. There are multiple strong motivations for the wide CTA energy range: the lowest energies provide access to the whole universe (avoiding significant gamma-gamma absorption on the extragalactic background light); the highest energies are needed to study the extreme accelerators which we know from direct cosmic-ray measurements are present in our galaxy; a wide energy range maximizes the chances of serendipitous detection of new source classes with unknown spectral characteristics, for example in the search for dark matter with an unknown WIMP mass; a wide energy range is key for discrimination between scenarios and to identify features. All objects which have been studied over a wide energy range with good signal to noise using current IACT arrays exhibit features in their gamma-ray spectra. Conversely, the narrow energy range and lower signal to noise measurements more typical of current generation instruments invariably result in spectra which are consistent with power-law forms. In the north, where the inner regions of the Galaxy are not visible, there will be a greater emphasis on extragalactic targets. Therefore, in the interest of optimization of the observatory, the northern CTA array will be implemented with only LSTs and MSTs.
Access to the full sky is necessary as many of the phenomena to be studied by CTA are rare and individual objects can be very important. For example, the most promising galaxy cluster, the brightest starburst galaxy and the only known gravitationally-lensed TeV source are located in the north. The inner Galaxy and the Galactic Center are key CTA targets and are located in the south. Full sky coverage ensures that extremely rare but critically important events (for example a Galactic supernova explosion, bright gravitational wave transient, or nearby gamma-ray burst) will be accessible to CTA.
Individual CTA telescopes will have Cherenkov cameras with wide field of view: >4.5º for the LSTs, >7º for the MSTs and >8º for the SSTs. The wide camera field serves a dual purpose: to provide contained shower images up to large impact distance (improving collection area and resolution) for on-axis gamma rays and to increase the gamma-ray field of view of the system as a whole. This characteristic of CTA is critical for the observation of very extended objects and regions of diffuse emission, as well as for surveys. Furthermore, the wide field provides reduced systematic errors, with a uniform response over regions much larger than the point-spread-function size (not always the case for current generation instruments).
The large telescope number (~100 in the south) and individual wide telescope fields of view result in a CTA collection area which is one or more orders of magnitude larger than current generation instruments at essentially all energies, with substantial benefits for imaging, spectroscopy and light-curve generation. Multi-square-kilometer collection area is essential at the highest energies where there is essentially zero background even in long exposures and sensitivity is limited by the collection of sufficient signal photons. For very short timescale phenomena, CTA is background free over much of its energy range and the large collection area is the key performance driver.
For events incident in the central parts of the CTA arrays, the number of recorded shower images will be large (>10) for all but the lowest energies. These high image multiplicities, combined with the contained nature of events and superior image information to existing instruments, provide excellent energy and angular resolution. A precision of 1 arc-minute on individual photons will be obtained for the upper end of the CTA energy range, the best resolution achieved anywhere above the X-ray domain.
The ability to rapidly respond to external alerts, and to rapidly issue its own alerts, is built into the CTA design. In particular the LSTs, where the energy range covered provides access to essentially the whole universe, are optimized for rapid movement, with a goal slewing time of 20 s (minimum requirement 50 s) to anywhere in the observable sky. A real-time analysis pipeline will enable the identification of significant gamma-ray activity in any part of the field of view and the issuing of alerts to other instruments within one minute.
The dramatic improvement in the point-source sensitivity of CTA with respect to current instruments is a consequence of the combination of improved background rejection power, increased collection area and improved angular resolution. The improved background rejection power is achieved primarily through high image multiplicity and is particularly important for the study of extended, low-surface brightness objects and for low-flux objects where deep exposures are required. Figure 1 compares the sensitivity and angular resolution of the CTA arrays to a selection of existing gamma-ray detectors.
Finally, the number of individual telescopes in the CTA arrays, and the ability to operate multiple subarrays independently, provides enormous flexibility of operation. CTA will operate with different pointing directions for different sub-systems, for example with the LSTs pointed to a distant active galaxy and the MSTs and SSTs observing a hard-spectrum Galactic source. Furthermore, small groups of MSTs or SSTs may be used to monitor up to ten variable sources simultaneously. The pointing pattern of the CTA telescopes may also be optimized for the purpose of surveying an extended region of arbitrary shape, for example the error box from a gravitational wave alert . 2) 3) Preliminary CTA performance curves are available publicly at. 4) Below we highlight two key aspects of the unique instrumental reach of CTA.
CTA will conduct a census of particle acceleration in our universe by performing surveys of the sky at unprecedented sensitivity at very high energies. Deep fields will be obtained for some key regions hosting prominent targets, while wider and shallower surveys will be conducted to build up unbiased population samples and to search for the unexpected. The combination of the wide CTA field of view with unprecedented sensitivity ensures that CTA can deliver surveys one to two orders of magnitude deeper than existing surveys early in the life of the observatory. Indeed over much of the sky and much of the energy range of CTA, no sensitive survey exists and CTA measurements will be revolutionary. The CTA surveys will open up discovery space in an unbiased way and generate legacy datasets of long-lasting value.
Figure 1: Comparisons of the performance of CTA with selected existing gamma-ray instruments. Top: differential energy flux sensitivities for CTA (south and north) for five standard deviation detections in five independent logarithmic bins per decade in energy. For the CTA sensitivities, additional criteria are applied to require at least ten detected gamma rays per energy bin and a signal/background ratio of at least 1/20. The curves for Fermi-LAT (Fermi- Large Area Telescope) and HAWC (High Altitude Water Cherenkov) Experiment are scaled by a factor of 1.2 to account for the different energy binning. The curves shown give only an indicative comparison of the sensitivity of the different instruments, as the method of calculation and the criteria applied are different. In particular, the definition of the differential sensitivity for HAWC is rather different due to the lack of energy reconstruction for individual photons in the HAWC analysis. — Bottom: angular resolution expressed as the 68% containment radius of reconstructed gamma rays (the resolution for CTA-North is similar). The sensitivity and angular resolution curves are based on the following references: Fermi-LAT [5)], HAWC [6)], H.E.S.S. [7)], MAGIC [8)], and VERITAS [9)]. The CTA curves represent the understanding of the performance of CTA at the time of completion of this document; for the latest CTA performance plots, see https://www.cta-observatory.org/science/cta-performance.
Short Timescale Capabilities
CTA is a uniquely powerful instrument for the exploration of the violent and variable universe. It has unprecedented potential both in terms of energy reach and sensitivity to short timescale phenomena. Figure 2 compares the sensitivity of CTA to that of Fermi-LAT as a function of observation time. CTA has four orders of magnitude better sensitivity to minute timescale phenomena at energies around 25 GeV. Even at variability timescales of 1 month, CTA will be a factor 100 more sensitive than Fermi-LAT. Only for emission which is stable over the full mission lifetime of Fermi are the sensitivities of the two instruments comparable in the lowest part of the CTA energy range. Instruments such as HAWC, which have sensitivity in the higher part of the CTA range, are also limited at short timescales by (relative to CTA) small collection areas, as well as low signal to noise.
Figure 2: Comparison of the sensitivities of CTA and Fermi-LAT in the energy range of overlap versus observation timescale. Differential flux sensitivities at three energies are compared. Adapted from [10)]. Note that the Pass 6 sensitivity is shown for Fermi-LAT and the CTA sensitivity is calculated using an early model of the arrays; thus, better sensitivities for both Fermi-LAT and CTA are now expected.
CTA will look at the sky in higher energy photons than ever measured before. In fact, the cosmic particle accelerators CTA will probe can reach energies inaccessible to man-made accelerators like the Large Hadron Collider at CERN. 11)
CTA’s unique capabilities will help us to address some of the most perplexing questions in astrophysics. CTA will seek to understand the impact of high-energy particles in the evolution of cosmic systems and to gain insight into the most extreme and unusual phenomena in the Universe. CTA will search for annihilating dark matter particles and deviations from Einstein’s theory of relativity and even conduct a census of particle acceleration in the Universe.
CTA’s unique capabilities will include:
• CTA will have unprecedented accuracy and will be 10 times more sensitive than existing instruments
• An energy resolution of 10 percent will improve CTA’s ability to look for spectral features and lines associated with the annihilation of dark matter particles
• Rapid slewing in as low as 20 seconds will allow CTA to catch gamma-ray bursts ‘in the act’ of exploding
• Energies as low as 20 GeV will allow CTA to probe transient and time-variable gamma-ray phenomena in the very distant universe with unprecedented precision
• Energies up to 300 TeV will push CTA beyond the edge of the known electromagnetic spectrum, providing a completely new view of the sky and allowing us to search for extreme particle accelerators
• A field of view of eight degrees will allow CTA to survey the sky much faster and measure very extended regions of gamma-ray emission
• An angular resolution approaching one arcminute will allow CTA to resolve many cosmic sources to understand how ultra-relativistic particles are distributed in and around these systems.
Electromagnetic Spectrum 12)
To understand gamma rays, it’s important to understand the electromagnetic spectrum. The light we see is actually just a small portion of the total amount of light that surrounds us. The electromagnetic spectrum is used to describe the entire range of light that exists.
Light is made up of waves of alternating electric and magnetic fields that can be measured by frequency (number of waves that pass by a point in one second) or wavelength (the distance from the peak of one wave to the next). The larger the frequency, the smaller the wavelength. The spectrum ranges from the very lowest frequencies of radio waves and microwaves; to the mid-range frequencies found in infrared, optical (visible) and ultraviolet light; to the very highest frequencies of X-rays and gamma rays. The frequency range of gamma rays is so vast that it doesn’t even have a well-defined upper limit. The gamma rays CTA will detect are about 10 trillion times more energetic than visible light.
Figure 3: The electromagnetic spectrum (image credit: CTA)
The electromagnetic spectrum provides scientists with a variety of ways to view the Universe. As seen in the Figures 4 and 5, telescopes detecting different frequencies of light provide different perspectives of the Milky Way and the Crab Nebula, providing a more complete picture of the phenomena they are studying. With its ability to view the highest-energy processes in the Universe, CTA will be a vital asset in improving our understanding of these phenomena.
Figure 5: The Crab Nebula in radio, infrared, visible ultraviolet, X-ray and gamma-ray wavelengths [Sources: Radio: NRAO/AI and M. Bietenholz, J. M. Uson, T. J. Cornwell; Infrared: NASA/JPL-Caltech/R. Gehrz (University of Minnesota); Visible: NASA, ESA, J. Hester and A. Loll (Arizona State University); Ultraviolet: NASA/Swift/E. Hoversten, PSU; X-ray: NASA/CXC/SAO/F. Seward et al.; Gamma-ray: NASA/DOE/Fermi LAT/ R. Buehler]
For example, supernova remnants – the giant explosion shells generated by dying stars – are suspected as accelerators of cosmic rays. CTA will have the resolution to identify specific regions of supernova remnants and probe the presence of high-energy cosmic rays, that serve as sources of gamma rays.
Most of the light we are used to seeing is emitted by hot objects and is known as thermal radiation. The hotter the source of this radiation, the higher the frequency of the light produced. However, it is not possible for objects to get hot enough to produce gamma rays; these must be produced by a non-thermal mechanism. The mechanisms often rely on the presence of high-energy subatomic particles that are produced by some kind of cosmic particle accelerator.
Accelerated particles develop in special environments where a small fraction of the particles can take on an “un-fair” share (or fraction) of the energy available. In such a system, a small number of particles can be accelerated to very close to the speed of light and carry a significant fraction of the energy available. Since energy is no longer shared roughly equally among particles – as is the case in a “normal” hot environment – these processes are referred to as non-thermal processes.
These special environments are usually associated with violent events such as explosions, outbursts or powerful jets of material produced close to the giant black holes at the center of galaxies. For this reason, gamma rays can be used to trace violent events in the universe.
CTA will be sensitive to the highest-energy gamma rays, making it possible to probe the physical processes at work in some of the most violent environments in the Universe. Although cosmic gamma rays cannot reach the earth’s surface, CTA can detect them from the ground using the subatomic particle cascades that they produce in the atmosphere. Charged particles in these cascades travel at very close to the speed of light and emit visible (mostly blue) light known as Cherenkov light. CTA’s large telescope mirrors and ultra-high-speed cameras can then collect and record the nanosecond flash of light so that the incoming gamma ray can be tracked back to its cosmic source.
CTA will be able to detect hundreds of objects in our galaxy, the Milky Way. These galactic sources will include the remnants of supernova explosions, the rapidly spinning ultra-dense stars known as pulsars and more normal stars in binary systems and large clusters. Beyond the Milky Way, CTA will detect star-forming galaxies and galaxies with supermassive black holes at their centers (active galactic nuclei or AGN) and, possibly, whole clusters of galaxies. The gamma rays detected with CTA may also provide a direct signature of dark matter, evidence for deviations from Einstein’s theory of relativity and definitive answers to the contents of cosmic voids, the empty space that exists between galaxy filaments in the Universe.
The CTA Consortium includes 1,500 members from more than 200 institutes in 31 countries. The scientists and engineers of the CTA Consortium devised the CTA concept more than a decade ago and have been the driving force behind its design. The Consortium has developed and detailed CTA’s key science goals (see “Science with the Cherenkov Telescope Array“) and will be responsible for the science analysis and publication of scientific results of the Key Science Projects, ensuring that CTA produces legacy data sets and data products for use by the entire community. Consortium member institutes will make in-kind contributions to CTA construction and will support array commissioning and science verification and science operations. 13)
Figure 6: Overview of countries participating in CTA's science goals and array design (image credit: CTA)
Figure 7: The CTA Observatory (CTAO) consists of two array site locations — one in Chile and one in La Palma — and three office locations — the CTAO Headquarters (interim) and Science Data Management Centre in Germany and the local office (and future site of the headquarters) in Italy (image credit: CTA)
Headquarters and Science Data Management Centre
On 13 June 2016, the CTA Observatory (CTAO) Council selected Bologna as the host site of the CTAO Headquarters and Berlin-Zeuthen for the Science Data Management Centre (SDMC), among five site candidates.
The Council, composed of shareholders from nine countries (Austria, Czech Republic, France, Germany, Italy, Japan, Spain, Switzerland and the United Kingdom) in consultation with associate members (Netherlands, South Africa and Sweden), made the decision after careful consideration of the proposals against criteria that included infrastructure, services and access requirements.
“We are grateful for all of the proposals put forward by the applicants. While each of the candidate sites were suitable options, the Council is confident that Bologna and Zeuthen will be well-equipped to support CTA’s long-term operations,” said Ulrich Straumann, Managing Director of the CTAO gGmbH.
The CTA Headquarters will be the central office responsible for the overall administration of Observatory operations. Approximately two dozen personnel will provide technical coordination and support, and the main administrative services for the governing bodies and users of the Observatory. The headquarters will be located within the Istituto Nazionale di Astrofisica (INAF) premises in a new building shared with the Bologna University Department of Physics and Astronomy. This location gives CTA a home in a word-class scientific environment with state‐of‐the-art facilities, in one of Italy’s most attractive and historic cultural centres.
The Science Data Management Centre will coordinate science operations and make CTA’s science products available to the worldwide community. An estimated 20 personnel will manage CTA’s science coordination including software maintenance and data processing for the Observatory, which is expected to generate approximately 100 petabytes (PB) of data by the year 2030. (One PB is equal to 1015 bytes of data). The SDMC will be located in a new building complex on the Deutsches Elektronen-Synchrotron (DESY) campus in Zeuthen, which is conveniently located just outside Berlin – one of Europe’s primary capital cities. This location provides extensive access to well-established infrastructure services and a powerful computing center.
CTA — the World’s Largest Ground-Based Gamma-Ray Observatory
The Cherenkov Telescope Array (CTA) will be a next-generation ground-based observatory for very high energy gamma-ray astronomy. It will consist of two arrays of dishes, a southern-hemisphere array near ESO’s Paranal Observatory and a northern array on the island of La Palma, Spain. Gamma-rays are emitted by some of the hottest and most powerful objects in the Universe, such as supermassive black holes and supernovae. 14)
CTA, with its large collecting area and wide sky coverage, will be the largest and most sensitive high-energy gamma-ray observatory in the world. The two arrays will detect gamma-rays with unprecedented accuracy and together will be 10 times more sensitive than existing instruments.
Three types of telescope are required to cover the full CTA energy range (20 GeV to 300 TeV). Over its core energy range (100 GeV to 10 TeV), CTA will employ 40 MSTs (Medium-Sized Telescopes) distributed over the two array sites, accompanied by 8 LSTs (Large-Sized Telescopes) and 70 SSTs (Small-Sized Telescopes), reaching below 100 GeV and above 10 TeV, respectively.
The current plan is that on completion, CTA will comprise 118 telescopes worldwide, with 19 dishes in the northern hemisphere and 99 dishes in the southern hemisphere. The northern hemisphere site is located at the Instituto de Astrofísica de Canarias Observatorio del Roque de los Muchachos on the island of La Palma in the Canary Islands. The southern site is located at ESO’s Paranal Observatory, around 10 km southeast of the Very Large Telescope. This is one of the driest and most isolated regions on Earth — an astronomical paradise. Integrating CTA into the existing Paranal–Armazones infrastructure will allow it to take advantage of ESO’s state-of-the-art facilities.
Figure 8: The CTA-North Site: Our Northern Eye on the High-Energy Universe. Capturing particle showers from a gamma ray that interacts with the Earth's atmosphere is a pretty big challenge. That's why CTA will use two arrays of telescopes to explore the entire night sky: one in the northern hemisphere (CTA-North) and one in the southern hemisphere (CTA-South). In this film, CTA-North Site Manager, Paolo Calisse, will introduce you to the northern site, which is located at the Roque de los Muchachos Observatory on La Palma, a Spanish island in the Canary Islands (video credit: CTA Consortium)
Detecting Cherenkov Light
The gamma rays that CTA will detect don’t make it all the way to the Earth’s surface. When gamma rays reach the earth’s atmosphere they interact with it, producing cascades of subatomic particles. These cascades are also known as air or particle showers. Nothing can travel faster than the speed of light in a vacuum, but light travels 0.03 percent slower in air. Thus, these ultra-high energy particles can travel faster than light in air, creating a blue flash of “Cherenkov light” (discovered by Russian physicist Pavel Cherenkov in 1934) similar to the sonic boom created by an aircraft exceeding the speed of sound. Although the light is spread over a large area (250 m in diameter), the cascade only lasts a few billionths of a second. It is too faint to be detected by the human eye but not too faint for CTA. CTA’s large mirrors and high-speed cameras will detect the flash of light and image the cascade generated by the gamma rays for further study of their cosmic sources. 15)
Figure 9: How will the Cherenkov Telescope Array explore the Universe at the highest energies? This science animation takes you through the process — from the emission of gamma rays by extreme sources and the collection of Cherenkov light by CTA on Earth to data analysis and discovery. This film is the first in a series of videos we will be releasing about CTA in 2020 (video credit: CTA)
These cascades are so rare (one gamma-ray photon per m2 per year from a bright source or one per m2 per century from a faint source), that CTA will be using more than 100 telescopes spread between two array sites in the northern and southern hemispheres to improve the chance of capturing them. The graphic below illustrates a potential layout of the telescope arrays in both the northern and southern hemispheres.
While the northern hemisphere array will be more limited in size and will focus on CTA’s low- and mid-energy ranges from 20 GeV to 20 TeV, the southern hemisphere array, with its prime view of the rich central region of our Galaxy, will span the entire energy range of CTA, covering gamma-ray energies from 20 GeV to 300 TeV. Three classes of telescope will be distributed in the northern and southern hemisphere based on their sensitivity: the Small-Sized Telescope (SST), Medium-Sized Telescope (MST), and Large-Sized Telescope (LST). Because the SSTs are tuned to be the most sensitive to detect high-energy gamma rays, they are more ideal for the southern site’s detection of higher-energy gamma rays, while the MSTs and LSTs will be installed on both sites. The below schematics and illustrate the proposed layouts of the northern and southern hemisphere arrays.
Figure 10: Planned CTA layouts in the northern and southern hemisphere sites (image credit: CTA Consortium)
Figure 11: Rendering of the Northern Hemisphere Site (image credit: Gabriel Pérez Diaz, IAC)
Figure 12: Rendering of the Southern Hemisphere Site (image credit: Gabriel Pérez Diaz, IAC / Marc-André Besel, CTAO)
CTA Telescopes and Technology
CTA is not the first ground-based gamma-ray detector, but it will be the most advanced of its kind. The current generation started yielding results in 2003 and increased the number of known gamma-ray-emitting objects from around 10 to more than 100. CTA will build on the advances pioneered by its predecessors (H.E.S.S., VERITAS and MAGIC) in order to expand this catalogue tenfold, detecting more than 1,000 new objects.
Three classes of telescope types are required to cover the full CTA energy range (20 GeV to 300 TeV). For its core energy range (100 GeV to 10 TeV), CTA is planning 40 Medium-Sized Telescopes distributed over both array sites. Furthermore, eight Large-Sized Telescopes and 70 Small-Sized Telescopes are planned to extend the energy range below 100 GeV and above a few TeV.
While the individual telescopes may vary in size and design, CTA telescopes will be constructed and will perform similarly. Each telescope will have a mount that allows it to rapidly point towards targets and will be comprised of a large segmented mirror to reflect the Cherenkov light to a high-speed camera that can digitize and record the image of the shower.
The telescope structures will stand between about 8 and 45 meters tall and weigh between 8 and 100 tons.
Large-Sized Telescope (LST): Because gamma rays with low energies produce only a small amount of Cherenkov light, telescopes with large mirrors are required to capture the images. The LST mirror will be 23 m in diameter and parabolic in shape. Its camera will use photomultiplier tubes (PMTs) and will have a field of view of about 4.5 degrees. The entire structure will weigh 50 tons but will be extremely nimble, with the goal to be able to re-position within 20 seconds. More about the LST.
Medium-Sized Telescope (MST): This will be CTA’s “workhorse” due to its sensitivity to the faint energy flux of gamma rays. The MST mirror will be 12 m in diameter and will have a camera that uses PMTs. Its large field of view of 7-8 degrees will enable the MST to take rapid surveys of the gamma-ray sky. More about the MST.
Small-Sized Telescope (SST): The SST is sensitive to the highest energy gamma rays, which come from our own galaxy. Since our galaxy is best observed from the southern hemisphere and the corresponding showers produce a large amount of Cherenkov light, the SSTs will outnumber all the other telescopes and will be spread out over several square kilometers in the southern hemisphere array only. The SST mirror will be about 4 m in diameter and will have a large field of view of about 9 degrees. More about the SST.
CTA Cameras: CTA will use more than 7,000 highly-reflective mirror facets (90 cm to 2 m in diameter) to focus light into the telescopes’ cameras. Once the mirrors reflect the light, the CTA cameras capture and convert it into data. Each telescope has its own variation of camera (see example of one of the proposed camera prototypes below), but the designs are all driven by the brightness and short duration of the Cherenkov light flash.
A Cherenkov light flash lasts only a few billionths of a second and is extremely faint. The cameras are sensitive to these faint flashes and use extremely fast exposures to capture the light. CTA will use both photomultiplier tubes (PMTs) and silicon photomultipliers (SiPMs) to convert the light into an electrical signal that is then digitized and transmitted.
Figure 13: Camera prototype undergoing testing at University of Leicester (image credit: Akira Okumura)
CTA development status
• June 12, 2020: During the week of 8 June 2020, at the remote biannual general meeting of the Large-Sized Telescope (LST) consortium, the CTA Observatory (CTAO) announced that the LST prototype, the LST-1, passed its Critical Design Review (CDR). It is the first CTA element to pass such a review and is one important step toward closing out the CDR and starting the process for the acceptance and handover of the first LST to the observatory, which is planned to occur in 2021 after the CTA ERIC has been established. 16)
- The CTAO Project Office held the CDR for the LST-1, beginning with the LST team’s submission of review documentation at the beginning of August 2019 and culminating in the review meeting in Munich in October. A CDR will be conducted for all CTA subsystems to verify that the detailed design has been successfully completed and satisfies the specified requirements.
- The LST team submitted more than 700 documents to the review panel, which was comprised of nine external experts and 17 from the CTAO systems engineering and software teams. The CDR was held in a very collaborative atmosphere, and the panel congratulated the LST team for the large amount of work they accomplished to prepare for the review and for promptly responding to the approximately 950 questions, comments and discrepancies the panel submitted before the meeting.
- The review identified both major and minor issues to be addressed by the CTAO and/or LST teams. The CTAO and LST project managers agreed that to pass the review, certain critical items needed to be addressed and a plan established to work on the remaining issues—that milestone has now been achieved. The work to close out all of the major action items will continue and, once all are addressed, the CDR will be declared closed before moving to acceptance by the CTAO.
Figure 14: LST-1 at Sunset, taken on 13 August 2019. The LST-1 waits for the sun to set on La Palma before getting to work (image credit: Otger Ballester (IFAE)
• 2018: The project to build CTA is well-advanced: the Medium-Sized Telescope prototype and all three proposed designs of the Small-Sized Telescope have achieved ‘first light,’ the Large-Sized Telescope prototype was inaugurated in October 2018 and the Schwarzschild-Couder Telescope prototype inauguration is in January 2019. An inter-governmental agreement for construction and subsequent operation of the observatory, for which a European Research Infrastructure Consortium (ERIC) is foreseen, is in preparation and the financial threshold is expected to be reached by the middle of 2018. 17)
- The timeline (Figure 15) illustrates the estimated timing for major project milestones through the beginning of 2020. In 2018, infrastructure conceptual planning for the northern hemisphere site is mostly complete, and the detailed designs and necessary permits for construction of the telescope foundations and infrastructure are being prepared. Construction will begin in 2019, if all goes as planned.
Figure 15: CTA has come a long way since its conception in 2005. This timeline illustrates CTA’s progress and the estimated timeline for major milestones yet to be completed in order for operations to begin in 2022 and array construction to end in 2025 (image credit: CTA Consortium)
Events and status of the CTA
• June 22, 2020: Between January and February 2020, the prototype Large-Sized Telescope (LST), the LST-1, observed the Crab Pulsar, the neutron star at the center of the Crab Nebula. The telescope, which is being commissioned on the CTA-North site on the island of La Palma in the Canary Islands, was conducting engineering runs to verify the telescope performance and adjust operating parameters. 18)
- Pulsars are very rapidly rotating and strongly magnetized neutron stars that emit light in the form of two beams, which can be observed from Earth only when passing our line of sight. While detecting the strong and steady emission or outbursts of gamma-ray sources with Imaging Atmospheric Cherenkov Telescopes (IACTs) has become routine, pulsars are much more challenging to detect due to their weak signals and the typical dominance of the foreground gamma-ray signal from the surrounding nebulae. Despite hundreds of observations hours by IACTs around the globe, only four pulsars emitting signals in the very high-energy gamma-ray regime have been discovered, so far. Now that the LST-1 has shown that it can detect the Crab pulsar, it joins the field of telescopes capable of detecting gamma-ray pulsars, validating the timestamping system and the low-energy performance of the telescope.
- “This milestone shows us that the LST-1 is already performing at an extraordinary level, detecting a challenging source in record time,” says Masahiro Teshima, Director of Max-Planck-Institute for physics in Munich and Principal Investigator of LST. “Pulsars are one of the key scientific targets of the LSTs, and it’s exciting to imagine what we’ll be able to achieve when the telescope is fully commissioned and operational.”
- The data set collected includes 11.4 hours from eight observation nights. Figure 17 shows the resulting phasogram, plotting the gamma-ray events as a function of the pulsar rotation phase. In the phase regions marked as P1 and P2, more gamma rays are expected as the Crab pulsar emits towards the Earth. The emission detected in all phases (marked green in Figure 17) is a mixture of different background contributions, including the irreducible steady emission from the Crab Nebula. The signal detected with the LST-1 (marked red in Figure 17) is undeniably significant for phase P2, while the signal during P1 is still marginal. The animation in Figure 18 highlights the pulse behavior of the source during the different phases.
Figure 16: Multiwavelength view of the Crab Nebula and the Crab pulsar – the bright spot at the center of the image (image credit: NASA, ESA, G. Dubner (IAFE, CONICET-University of Buenos Aires) et al.; A. Loll et al.; T. Temim et al.; F. Seward et al.; VLA/NRAO/AUI/NSF; Chandra/CXC; Spitzer/JPL-Caltech; XMM-Newton/ESA; Hubble/STScI)
Figure 17: Phasogram of the Crab Pulsar as measured by the LST-1. The pulsar is known to emit pulses of gamma rays during phases P1 and P2. The shown significance is calculated considering source emission from those phases (in red) and background events from phases in grey (image credit: LST Collaboration)
• On 1 June 2020, scientists from the Cherenkov Telescope Array (CTA) Consortium announced at the 236th meeting of the American Astronomical Society (AAS) that they have detected gamma rays from the Crab Nebula using a prototype telescope proposed for CTA, the prototype Schwarzschild-Couder Telescope (pSCT), proving the viability of the novel telescope design for use in gamma-ray astrophysics. 19) 20)
- “The Crab Nebula is the brightest steady source of TeV, or very-high-energy, gamma rays in the sky, so detecting it is an excellent way of proving the pSCT technology,” said Justin Vandenbroucke, Associate Professor, University of Wisconsin. “Very-high-energy gamma rays are the highest energy photons in the universe and can unveil the physics of extreme objects including black holes and possibly dark matter.”
- Detecting the Crab Nebula with the pSCT is more than just proof-positive for the telescope itself. It lays the groundwork for the future of gamma-ray astrophysics. “We’ve established this new technology, which will measure gamma rays with extraordinary precision, enabling future discoveries,” said Vandenbroucke. “Gamma-ray astronomy is already at the heart of the new multi-messenger astrophysics, and the SCT technology will make it an even more important player.”
- The use of secondary mirrors in gamma-ray telescopes is a leap forward in innovation for the relatively young field of very-high-energy gamma-ray astronomy, which has moved rapidly to the forefront of astrophysics. “Just over three decades ago, TeV gamma rays were first detected in the universe, from the Crab Nebula, on the same mountain where the pSCT sits today,” said Vandenbroucke. “That was a real breakthrough, opening a cosmic window with light that is a trillion times more energetic than we can see with our eyes. Today, we’re using two mirror surfaces instead of one, and state-of-the-art sensors and electronics to study these gamma rays with exquisite resolution.”
Figure 19: Animation showing 18 gamma-ray events from the Crab Nebula detected with the pSCT telescope (image credit: CTA/SCT consortium)
- “The initial pSCT Crab Nebula detection was made possible by leveraging key simultaneous observations with the co-located VERITAS (Very Energetic Radiation Imaging Telescope Array System) observatory. We have successfully evolved the way gamma-ray astronomy has been done during the past 50 years, enabling studies to be performed in much less time,” said Wystan Benbow, Director, VERITAS. “Several future programs will particularly benefit, including surveys of the gamma-ray sky, studies of large objects like supernova remnants, and searches for multi-messenger counterparts to astrophysical neutrinos and gravitational wave events.”
Figure 20: Sky map recorded with the pSCT over a region centered on the Crab Nebula, detection of the Crab Nebula marked at center (image credit: CTA/SCT consortium)
Figure 21: Histogram showing the detection of gamma-ray events from the Crab Nebula, with NOFF representing background and NON representing a combination of signal and background (image credit: CTA/SCT consortium)
- Located at the Fred Lawrence Whipple Observatory in Amado, Arizona—the largest field site of the Center for Astrophysics | Harvard & Smithsonian—the pSCT was inaugurated in January 2019 and saw first light the same week. After a year of commissioning work, scientists began observing the Crab Nebula in January 2020, but the project has been underway for more than a decade.
- “We first proposed the idea of applying this optical system to TeV gamma-ray astronomy nearly 15 years ago, and my colleagues and I built a team in the US and internationally to prove that this technology could work,” said Prof. Vladimir Vassiliev, Principal Investigator, pSCT. “What was once a theoretical limit to this technology is now well within our grasp, and continued improvements to the technology and the electronics will further increase our capability to detect gamma rays at resolutions and rates we once only ever dreamed of.”
- The pSCT was made possible by the contributions of thirty institutions and five critical industry partners across the United States, Italy, Germany, Japan, and Mexico, and by funding through the U.S National Science Foundation Major Research Instrumentation Program.
- “That a prototype of a future facility can yield such a tantalizing result promises great things from the full capability, and exemplifies NSF’s interest in creating new possibilities that can enable a project to attract wide-spread support,” said Nigel Sharp, Program Manager, National Science Foundation.
- The SCT is being proposed to cover the core of CTA’s energy range (around 80 GeV – 50 TeV). The SCT’s two-mirror optical system is designed to better focus the light for greater imaging detail and improved detection of faint sources. A total of 40 telescopes (25 in the southern hemisphere and 15 in the northern hemisphere) are planned to cover this energy range. “CTA has been exploring the dual-mirror technology since the very beginning of the project with the Small-Sized Telescope prototypes and the SCT for the Medium-Sized Telescope,” says Federico Ferrini, Managing Director of the CTA Observatory. “The results obtained by the pSCT team re-confirm the advantages of advancing the technology for Cherenkov astronomy and the great potential it will bring to CTA.”
- “The pSCT, and its innovations, are pathfinding for the future CTA, which will detect gamma-ray sources at around 100 times faster than VERITAS, which is the current state of the art,” said Benbow. “We have demonstrated that this new technology for gamma-ray astronomy unequivocally works. The promise is there for this groundbreaking new observatory, and it opens a tremendous amount of discovery potential.”
Figure 22: Guests of the pSCT inauguration in January 2019 gather in front of the telescope (image credit: Deivid Ribeiro, Columbia University)
About the pSCT: The SCT optical design was first conceptualized by U.S. members of CTA in 2006, and the construction of the pSCT was funded in 2012. Preparation of the pSCT site at the base of Mt. Hopkins in Amado, AZ, began in late 2014, and the steel structure was assembled on site in 2016. The installation of the pSCT’s 9.7-m primary mirror surface — consisting of 48 aspheric mirror panels — occurred in early 2018, and was followed by the camera installation in May 2018 and the 5.4-m secondary mirror surface installation — consisting of 24 aspheric mirror panels — in August 2018. Scientists opened the telescope’s optical surfaces and observed first light in January 2019. It began scientific operations in January 2020. The SCT is based on a 114 year-old two-mirror optical system first proposed by Karl Schwarzschild in 1905, but only recently became possible to construct due to the essential research and development progress made at the Brera Astronomical Observatory, the Media Lario Technologies Incorporated and the Istituto Nazionale di Fisica Nucleare, all located in Italy. pSCT operations are funded by the National Science Foundation and the Smithsonian Institution.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).