A new facility to push forward very high-energy gamma-ray astronomy.
In 2004, as the telescopes of the High Energy Stereoscopic System (HESS) were starting to point towards the skies, there were perhaps 10 astronomical objects that were known to produce very high-energy (VHE) gamma rays – and exactly which 10 was subject to debate. Now, in 2012, well in excess of 100 VHE gamma-ray objects are known and plans are under way to take observations to a new level with the much larger Cherenkov Telescope Array.
VHE gamma-ray astronomy covers three decades in energy, from a few tens of giga-electron-volts to a few tens of tera- electron-volts. At these high energies, even the brightest astronomical objects have fluxes of only around 10–11 photons cm–2 s–1, and the inevitably limited detector-area available to satellite-based instruments means that their detection from space requires unfeasibly long exposure times. The solution is to use ground-based telescopes, although at first sight this seems improbable, given that no radiation with energies above a few electron-volts can penetrate the Earth’s atmosphere.
The possibility of doing ground-based gamma-ray astronomy was opened up in 1952 when John Jelley and Bill Galbraith measured brief flashes of light in the night sky using basic equipment sited at the UK Atomic Energy Research Establishment in Oxfordshire – then, as now, not famed for its clear skies (The discovery of air-Cherenkov radiation). This confirmed Blackett’s suggestion that cosmic rays, and hence also gamma rays, contribute to the light intensity of the night sky via the Cherenkov radiation produced by the air showers that they induce in the atmosphere. The radiation is faint – constituting about one ten-thousandth of the night-sky background – and each flash is only a few nanoseconds in duration. However, it is readily detectable with suitable high-speed photodetectors and large reflectors. The great advantage of this technique is that the effective area of such a telescope is equivalent to the area of the pool of light on the ground, some 104 m2.
These observations can help in answering fundamental physics questions concerning the nature of both dark matter and gravity
Early measurements of astronomical gamma rays using this method were difficult to make because there was no method of distinguishing the gamma-ray-induced Cherenkov radiation from that produced by the more numerous cosmic-ray hadrons. However, in 1985 Michael Hillas at Leeds University showed that fundamental differences in the hadron- and photon-initiated air showers would lead to differences in the shapes of the observed flashes of Cherenkov light. Applying this technique, the Whipple telescope team in Arizona made the first robust detection of a VHE gamma-ray source – the Crab Nebula – in 1989. When his technique was combined with the arrays of telescopes developed by the HEGRA collaboration and the high-resolution cameras of the Cherenkov Array at Themis, the imaging atmospheric Cherenkov technique was well and truly born.
The current generation of projects based on this technique includes not only HESS, in Namibia, but also the Major Atmospheric Gamma-Ray Imaging Cherenkov (MAGIC) project in the Canary Islands, the Very Energetic Radiation Imaging Telescope Array System (VERITAS) in Arizona and CANGAROO, a collaborative project between Australia and Japan, which has now ceased operation.
These telescopes have revealed a wealth of phenomena to be studied. They have detected the remains of supernovae, binary star systems, highly energetic jets around black holes in distant galaxies, star-formation regions in our own and other galaxies, as well as many other objects. These observations can help not only with understanding more about what is going on inside these objects but also in answering fundamental physics questions concerning, for example, the nature of both dark matter and gravity.
The field is now reaching the limit of what can be done with the current instruments, yet the community knows that it is observing only the “tip of the iceberg” in terms of the number of gamma-ray sources that are out there. For this reason, some 1000 scientists from 27 countries around the world have come together to build a new instrument – the Cherenkov Telescope Array (CTA).
The Cherenkov Telescope Array
The aim of the CTA consortium is to build two arrays of telescopes – one in the northern hemisphere and one in the southern hemisphere – that will outperform current telescope systems in a number of ways. First, the sensitivity will be a factor of around 10 better than any current array, particularly in the “core” energy range around 1 TeV. Second, it will provide an extended energy range, from a few tens of giga-electron-volts to a few hundred tera-electron-volts. Third, its angular resolution at tera-electron-volt energies will be of the order of one arc minute – an improvement of around a factor of four on the current telescope arrays. Last, its wider field of view will allow the array to survey the sky some 200 times faster at 1 TeV.
This unprecedented performance will be achieved using three different telescope sizes, covering the low-, intermediate- and high-energy regimes, respectively. The larger southern-hemisphere array is designed to make observations across the whole energy range. The lowest-energy photons (20–200 GeV) will be detected with a few large telescopes of 23 m diameter. Intermediate energies, from about 200 GeV to 1 TeV, will be covered with some 25 medium-size telescopes of 12 m diameter. Gamma rays at the highest energies (1–300 TeV) produce so many Cherenkov photons that they can be easily seen with small (4–6 m diameter) telescopes. These extremely energetic photons are rare, however, so a large area must be covered on the ground (up to 10 km2), needing as many as 30 to 70 small telescopes to achieve the required sensitivity. The northern-hemisphere array will cover only the low and intermediate energy ranges and will focus on observations of extragalactic objects.
Being both an astroparticle-physics experiment and a true astronomical observatory, with access for the community at large, the CTA’s science remit is exceptionally broad. The unifying principle is that gamma rays at giga- to tera-electron-volt energies cannot be produced thermally and therefore the CTA will probe the “non-thermal” universe.
Gamma rays can be generated when highly relativistic particles – accelerated, for example, in supernova shock waves – collide with ambient gas or interact with photons and magnetic fields. The flux and energy spectrum of the gamma rays reflects the flux and spectrum of the high-energy particles. They can therefore be used to trace these cosmic rays and electrons in distant regions of the Galaxy or, indeed, other galaxies. In this way, VHE gamma rays can be used to probe the emission mechanisms of some of the most powerful astronomical objects known and to probe the origin of cosmic rays.
VHE gamma rays can also be produced in a top-down fashion by decays of heavy particles such as cosmic strings or the hypothetical dark-matter particles. Large dark-matter densities that arise from the accumulation of the particles in potential wells, such as near the centres of galaxies, might lead to detectable fluxes of gamma rays, especially given that the annihilation rate – and therefore the gamma-ray flux – is proportional to the square of the density. Slow-moving dark-matter particles could give rise to a striking, almost mono-energetic photon emission.
The discovery of such line emission would be conclusive evidence for dark matter, and the CTA could have the capability to detect gamma-ray lines even if the cross-section is “loop-suppressed”, which is the case for the most popular candidates of dark matter, i.e. those inspired by the minimal supersymmetric extensions to the Standard Model and models with extra dimensions, such as Kaluza-Klein theory. Line radiation from these candidates is not detectable by current telescopes unless optimistic assumptions about the dark-matter density distribution are made. The more generic continuum contribution (arising from pion production) is more ambiguous but with its curved shape it is potentially distinguishable from the usual power-law spectra produced by known astrophysical sources.
It is not only the mechanisms by which gamma rays are produced that can provide useful scientific insights. The effects of propagation of gamma rays over cosmological distances can also lead to important discoveries in astrophysics and fundamental physics. VHE gamma rays are prone to photon–photon absorption on the extragalactic background light (EBL) over long distances, and the imprint of this absorption process is expected to be particularly evident in the gamma-ray spectra from active galactic nuclei (AGN) and gamma-ray bursts. The EBL is difficult to measure because of the presence of foreground sources of radiation – yet its spectrum reveals information about the history of star formation in the universe. Already, current telescopes detect more gamma rays from AGN than might have been expected in some models of the EBL, but understanding of the intrinsic spectra of AGN is limited and more measurements are needed.
Building the CTA
How to build this magnificent observatory? This is the question currently preoccupying the members of the CTA consortium. There is much experience and know-how within the consortium of building VHE gamma-ray telescopes around the world but nonetheless challenges remain. Foremost is driving down the costs of components while also ensuring reliability. It is relatively easy to repair and maintain four or five telescopes, such as those found in the current arrays, but maintaining 60, 70 or even 100 presents difficulties on a different scale. Technology is also ever changing, particularly in light detection. The detector of choice for VHE gamma-ray telescopes has until now been the photomultiplier tube – but these are bulky, relatively expensive and have low quantum-efficiency. Innovative telescope designs, such as dual-mirror systems, might allow the exploitation of newer, smaller detectors such as silicon photodiodes, at least on some of the telescopes. Mirror technologies are another area of active research because the CTA will require a large area of robust, easily reproducible mirrors.
The CTA is currently in its preparatory phase, funded by the European Union Seventh Framework Programme and by national funding agencies. Not only are many different approaches to telescope engineering and electronics being prototyped to enable the consortium to choose the best possible solution, but organizational issues, such as the operation of the CTA as an observatory, are also under development. It is hoped that building of the array will commence in 2014 and that it will become the premier instrument in gamma-ray astronomy for decades to come. Many of its discoveries will no doubt bring surprises, as have the discoveries of the current generation of telescopes. There are exciting times ahead.
• For more about the CTA project, see www.cta-observatory.org.
