Cherenkov Telescope Array CTA ( An advanced facility for high energy gamma rays astronomy )

Cherenkov Telescope Array CTA: an advanced facility for ground-based high-energy gamma-ray astronomy

Abstract Ground-based gamma-ray astronomy has had a major breakthrough with impressive results obtained systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics,particle physics and cosmology

.CTA is an international initiative to build  next generation instrument, with a factor of 5–10 improvement in sensitivity in the 100 GeV–10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. 

CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA. Keywords Ground based gamma ray astronomy· Next generation Cherenkov telescopes·Design concepts

 Executive summary

The Cherenkov Telescope Array (CTA) will explore in depth our Universe in very high energ y gamma-raysandinvestigatecosmicprocessesleadingtorelativistic particles, in close cooperation with observatories of other wavelength ranges of the electromagnetic spectrum, and those using cosmic rays and neutrinos. Besides guaranteed high-energy astrophysics results, CTA will have a large discovery potential in key areas of astronomy, astrophysics and fundamental physics research. These include the study of the origin of cosmic rays and their impactontheconstituentsoftheUniversethroughtheinvestigationofgalactic particle accelerators, the exploration of the nature and variety of black hole particle accelerators through the study of the production. extending well beyond its European roots.

 CTA will consist of two arrays of Cherenkov telescopes, which aim to: (a) increase sensitivity by another order of magnitude for deep observations around 1 TeV, (b) boost significantly the detection area and hence detection rates, particularly important for transient phenomena and at the highest energies, (c) increase the angular resolution and hence the ability to resolve the morphology of extended sources, (d) provide uniform energy coverage for photons from some tens of GeV to beyond 100 TeV, and (e) enhance the sky survey capability, monitoring capability and flexibility of operation.

 CTA will be operated as a proposal-driven open observatory, with a Science Data Centre providing transparent access to data, analysis tools and user training. To view the whole sky, two CTA sites are foreseen. The main site will be in the southern hemisphere, given the wealth of sources in the central region of ourGalaxyandtherichnessoftheirmorphologicalfeatures.AsecondcomplementarynorthernsitewillbeprimarilydevotedtothestudyofActiveGalactic Nuclei (AGN) and cosmological galaxy and star formation and evolution. The performance and scientific potential of arrays of Cherenkov telescopes have been studied in significant detail, showing that the performance goals can be reached. What remains to be decided is the exact layout of the telescope array. Ample experience exists in constructing and operating telescopes of the 12-m class (H.E.S.S., VERITAS).

 Telescopes of the 17-m class are operating (MAGIC) and one 28-m class telescope is under construction (H.E.S.S. II). These telescopes will serve as prototypes for CTA. The structural and optical properties of such telescopes are well understood, as many have been built for applications from radio astronomy to solar power installations. The fast electronics needed in gamma ray astronomy to capture the nanosecond-scale Cherenkov pulses have long been mastered, well before such electronics became commonplace with the Gigahertz transmission and processing used today in telephony, internet, television, and computing.
Exp Astron (2011) 32:193–316 195

The extensive experience of members of the consortium in the area of conventional photomultiplier tubes (PMTs) provides a solid foundation for the design of cameras with an optimal cost/performance ratio. Consequently, the base-line design relies on conventional PMTs. Advanced photon detectors with improved quantum efficiency are under development and test and may well be available when the array is constructed. In short, all the technical solutions needed to carry out this project exist today. 

The main challenge lies in the industrialisation of all aspects of the production and the exploitation of economies of scale. Given the large amounts of data recorded by the instrument and produced by computer simulations of the experiment, substantial efforts in e-science and grid computing are envisaged to enable efficient data processing. Some of the laboratories involved in CTA are Tier 1 and 2 centres on the LHC computinggridandtheCosmogrid.SimulationandanalysispackagesforCTA are developed for the grid. The consortium has set up a CTA-Virtual Organisation within the EGEE project (Enabling Grids for E-sciencE; funded by the European Union) for use of grid infrastructure and the sharing of computing resources,whichwillfacilitateworldwidecollaborationforsimulationsandthe processing and analysis of scientific data.

 Unlike current ground-based gamma-ray instruments, CTA will be an open observatory, with a Science Data Centre (SDC) which provides preprocessed data to the user, as well as the tools necessary for the most common analyses. The software tools will provide an easy-to-use and well-defined access to data from this unique observatory. CTA data will be accessible through the Virtual Observatory, with varying interfaces matched to different levels of expertise. The required toolkit is being developed by partners with experience in SDC management from, for example, the INTEGRAL space mission. Experiments in astroparticle physics have proven to be an excellent training ground for young scientists, providing a highly interdisciplinary work environment with ample opportunities to acquire not only physics skills but also to learn data processing and data mining techniques, programming of complex control and monitoring systems and design of electronics. Further, the environment of the large multi-national CTA Collaboration, working across international borders, ensures that presentation skills, communication ability and management and leadership proficiency are enhanced. Young scientists frequently participate in outreach activities and, thus, hone also their skills in this increasingly important area. With its training and mobility opportunities for young scientists, CTA will have a major impact on society.

The spectacular astrophysics results from the current Cherenkov instruments have generated considerable interest in both the astrophysics and particle physics communities and have created the desire for a next-generation,more sensitive and more flexible facility, able to serve a larger community of users. The proposed CTA2 is a large array of Cherenkov telescopes of different sizes, based on proven technology and deployed on an unprecedented scale. It will allow significant extension of our current knowledgeinhigh-energyastrophysics.CTAisanewfacility,withcapabilities well beyond those of conceivable upgrades of existing instruments such as H.E.S.S., MAGIC or VERITAS. The CTA project unites the main research groups in this field in a common strategy, resulting in an unprecedented convergence of efforts, human resources, and know-how. Interest in and support for the project is coming from scientists in Europe, America, Asia and Africa, all of whom wish to use such a facility for their research and are willing to contribute to its design and construction. CTA will offer worldwide unique opportunities to users with varied scientific interests. The number of11 GeV=109 eV; 1 TeV=1012 eV; 1 PeV=1015 eV. 2CTA was first publicly presented to an ESFRI panel in Autumn 2005.

in particular young scientists working in the still evolving field of gamma-ray astronomy is growing at a steady rate,drawing from other fields such as nuclear and particle physics. In addition, there is increased interest by other parts of the astrophysical community, ranging from radio to X-ray and satellite-based gamma-ray astronomers. CTA will, for the first time in this field, provide open access via targeted observation proposals and generate large amounts of public data, accessible using Virtual Observatory tools. CTA aims to become a cornerstone in a networked multi-wavelength, multi-messenger exploration of the high-energy non-thermal universe.

Radiation at gamma-ray energies differs fundamentally from that detected at lowerenergiesandhencelongerwavelengths:GeVtoTeVgamma-rayscannot conceivably be generated by thermal emission from hot celestial objects. The energy of thermal radiation reflects the temperature of the emitting body, and apart from the Big Bang there is and has been nothing hot enough to emit such gamma-rays in the known Universe. Instead, we find that highenergy gamma-rays probe a non-thermal Universe, where other mechanisms allow the concentration of large amounts of energy onto a single quantum of radiation. In a bottom-up fashion, gamma-rays can be generated when highly relativistic particles—accelerated for example in the gigantic shock waves of stellar explosions—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 our own Galaxy or even in other galaxies. High-energy gamma-rays can also be produced in a top-down fashion by decays of heavy particles such as hypothetical dark matter particles or cosmic strings, both of which might be relics of the Big Bang. Gamma-raysthereforeprovidea window on thediscovery ofthe nature and constituents of dark matter. High-energy gamma-rays, as argued above, can be used to trace the populations of high-energy particles in distant regions of our own or in other galaxies. Meandering in interstellar magnetic fields, cosmic rays will usually not reach Earth and thus cannot be observed directly. 

Those which do arrive have lost all directional information and cannot be used to pinpoint their sources, except for cosmic-rays of extreme energy >1018 eV. However, such high-energy particle populations are an important aspect of the dynamics of galaxies. Typically, the energy content in cosmic rays equals the energies in magnetic fields or in thermal radiation.The pressuregeneratedbyhigh-energy particles drives galactic outflows and helps balance the gravitational collapse.

Astronomy with high-energy gamma-rays is so far the only way to directly probe and image the cosmic particle accelerators responsible for these particle populations, in conjunction with studies of the synchrotron radiation resulting form relativistic electrons moving in magnetic fields and giving rise to non-thermal radio and X-ray emission.

A first glimpse of the astrophysical sources of gamma-rays
The first images of the Milky Way in VHE gamma-rays have been obtained in the last few years. These reveal a chain of gamma-ray emitters situated along the Galactic equator demonstrating that sources of highenergyradiationareubiquitousinourGalaxy.Sourcesofthisradiationinclude supernova shock waves, where presumably atomic nuclei are accelerated and generate the observed gamma-rays. Another important class of objects are “nebulae” surrounding pulsars, where giant rotating magnetic fields give rise to a steady flow of high-energy particles. Additionally, some of the objects discovered to emit at such energies are binary systems, where a black hole or a pulsar orbits a massive star. Along the elliptical orbit, the conditions for particle acceleration vary and hence the intensity of the radiation is modulated with the orbital period. These systems are particularly interesting in that they enable the study of how particle acceleration processes respond to varying ambient conditions. One of several surprises was the discovery of “dark sources”, objects which emit VHE gamma rays, but have no obvious counterpart in other wavelength regimes. In other words, there are objects in the Galaxy which might in fact be only detectable in high-energy gamma-rays. Beyond our Galaxy, many extragalactic sources of high-energy radiation have been discovered, located in active galaxies, where a super-massive black hole at the centre of the galaxy is fed by a steady stream of gas and is releasing enormous amounts of energy. Gamma-rays are believed to be emitted from the vicinity of these black holes, allowing the study of the processes occurring in this violent and as yet poorly understood environment.

The recent breakthroughs in VHE gamma-ray astronomy were achieved with ground-based Cherenkov telescopes. When a VHE gamma-ray enters the atmosphere, it interacts with atmospheric nuclei and generates a shower of secondary electrons, positrons and photons. Moving through the atmosphere at speeds higher than the speed of light in air, these electrons and positrons emit a beam of bluishlight,theCherenkovlight.Fornearverticalshowersthis Cherenkov light illuminates a circle with a diameter of about 250 m on the ground. For large zenith angles the area can increase considerably. This light can be captured with optical elements and be used to image the shower, which vaguelyresemblesashootingstar.Reconstructingtheshoweraxisinspaceand tracing it back onto the sky allows the celestial origin of the gamma-ray to be determined.Measuringmanygamma-raysenablesanimageofthegamma-ray sky, such as that shown in Fig. 2, to be created. Large optical reflectors with areas in the 100 m2 range and beyond are required to collect enough light, and the instruments can only be operated in dark nights at clear sites. With Cherenkov telescopes, the effective area of the detector is about the size of the Cherenkov pool at ground.

 As this is a circle with 250-m diameter this is about 105× larger than the size that can be achieved with satellite-based detectors. Therefore much lower fluxes at higher energies can be investigated with Cherenkov Telescopes, enabling the study of short time scale variability. The Imaging Atmospheric Cherenkov Technique was pioneered by the Whipple Collaboration in the United States. After more than 20 years of development, the Crab Nebula, the first source of VHE gamma-rays, was discovered in 1989. The Crab Nebula is among the strongest sources of very high energy gamma-rays, and is often used as a “standard candle”. Modern instruments, using multiple telescopes to track the cascades from different perspectives and employing fine-grained photon detectors for improved imaging, can detect sources down to 1% of the flux of the Crab Nebula. Finelypixellated imaging was first employed in the French CAT telescope [2], and the use of “stereoscopic” telescope systems to provide images of the cascade from different viewing points was pioneered by the European HEGRA IACT system [3]. For summaries of the achievements in recent years and the science case for a next-generation very high energy gamma ray observatory 

The High Energy Stereoscopic System (H.E.S.S.) project was awarded the Descartes Research Prize of the European Commission for offering “A new glimpse at the highest-energy Universe”. Together with the instruments MAGIC and VERITAS (in the northern hemisphere) and CANGAROO (in the southern hemisphere), a new wavelength domain was opened for astronomy, the domain of very high energy gamma-rays with energies between about 100 GeV and about 100 TeV, energies which are a million million times higher than the energy of visible light. At lower energies, in the GeV domain, the launch of a new generation of gamma-ray telescopes (like AGILE, but in particular Fermi, which was launched in 2008) has opened a new era in gamma-ray discoveries. The Large AreaTelescope(LAT),themaininstrumentonboardFermi,issensitive togamma-rayswithenergiesintherangefrom20MeVtoabout 100GeV.The energy range covered by CTA will smoothly connect to that of Fermi-LAT and overlap with that of the current generation of ground based instruments and extends to the higher energies, while providing an improvement in both sensitivity and angular resolution.

The CTA science drivers

The aims of the CTA can be roughly grouped into three main themes, serving as key science drivers: 1. Understanding the origin of cosmic rays and their role in the Universe 2. Understandingthenatureandvarietyofparticleaccelerationaroundblack holes 3. Searching for the ultimate nature of matter and physics beyond the Standard Model Theme 1 comprises the study of the physics of galactic particle accelerators, such as pulsars and pulsar wind nebulae, supernova remnants, and gammaray binaries. It deals with the impact of the accelerated particles on their environment (via the emission from particle interactions with the interstellar medium and radiationfields),and thecumulativeeffectsseenatvariousscales, from massive star forming regions to starburst galaxies. Theme2concernsparticleaccelerationnearsuper-massiveandstellar-sized black holes. Objects of interest include microquasars at the Galactic scale, and blazars,radiogalaxiesandotherclassesofAGNthatcanpotentiallybestudied in high-energy gamma rays. The fact that CTA will be able to detect a large number of these objects enables population studies which will be a major step forward in this area. Extragalactic background light (EBL), Galaxy clusters and Gamma Ray Burst (GRB) studies are also connected to this field. Finally, Theme 3 covers what can be called “new physics”, with searches for dark matter through possible annihilation signatures, tests of Lorentz invariance, and any other observational signatures that may challenge our current understanding of fundamental physics. CTA will be able to generate significant advances in all these areas.

 Details of the CTA science case

We conclude this chapter with a few examples of physics issues that could be significantly advanced with an instrument like CTA. The list is certainly not exhaustive. The physics of the CTA is being explored in detail by many scientists and their findings indicate the huge potential for numerous interesting discoveries with CTA.
 A tenet of high-energy astrophysics is that cosmic rays (CRs) are accelerated intheshocksofsupernovaexplosions.However,whileparticleaccelerationup to energies well beyond 1014 eV has now clearly been demonstrated with the current generation of instruments, it is by no means proven that supernovae accelerate the bulk of cosmic rays. The large sample of supernovae which will be observable with CTA—in some scenarios several hundreds of objects— and in particular the increased energy coverage at lower and higher energies, will allow sensitive tests of acceleration models and determination of their parameters. Improved angular resolution (arcmin) will help to resolve fine structures in supernova remnants which are essential for the study of particle acceleration and particle interactions. Pulsar wind nebulae surrounding the pulsars (created in supernova explosions) are another abundant source of high-energy particles, including possibly high-energy nuclei. Energy conversion within pulsar winds and the interaction of the wind with the ambient mediumandthesurroundingsupernovashellchallengecurrentideasinplasma physics. The CR spectrum observed near the Earth can be described by a pure power law up to an energy of a few PeV, where it slightly steepens. The feature is called the “knee”. The absence of other features in the spectrum suggests that, if supernova remnants (SNRs) are the sources of galactic CRs, they must be able to accelerate particles at least up to the knee. For this to happen, the acceleration in diffusive shocks has to be fast enough for particles to reach PeV energies before the SNR enters the Sedov phase, when the shock slows down and consequently becomes unable to confine the highest energy CRs Since the initial free expansion velocity of SNRs does not vary much from object to object, only the amplification of magnetic fields can increase the acceleration rate to the required level. Amplification factors of 100–1,000 compared to the interstellar medium value and small diffusioncoefficientsareneeded.Thenon-lineartheoryofdiffusiveshock acceleration suggests that such an amplification of the magnetic field might be induced by the CRs themselves, and high resolution X-ray observations of SNR shocks seem to support this scenario, though their interpretation is debated.Thus,anaccuratedeterminationoftheintensityofthemagneticfieldat the shock is of crucial importance for disentangling the origin of the observed gamma-ray emission and understanding the way diffusive shock acceleration works.

EvenifaSNRcanbedetectedbyCherenkovtelescopesduringasignificant fraction of its lifetime (up to several 104 years), it can make 1015 eV CRs only for a much shorter time (several hundred years), due to the rapid escape of PeV particles from the SNR. This implies that the number of SNRs which have currently a gamma-ray spectrum extending up to hundreds of TeV is very roughly of the order of ∼10. The actual number of detectable objects will depend on the distance and on the density of the surrounding interstellar medium. The detection of such objects (even a few of them) would be extremely important, as it would be clear evidence for the acceleration of CRs up to PeV energies in SNRs. A sensitive scan of the galactic plane with CTA wouldbeanidealwayofsearchingforthesesources.Ingeneral,thespectraof radiating particles (both electrons and protons) and therefore also the spectra of gamma-ray radiation, should show characteristic curvature, reflecting acceleration at CR modified shocks. However, to see such curvature, one needs a coverage of a few decades in energy, far from the cutoff region.

CTA will providethiscoverage.IfthegeneralpictureofSNRevolutiondescribedabove iscorrect,thepositionofthecutoffinthegamma-rayspectrumdependsonthe age of the SNR and on the magnetic field at the shock. A study of the number of objects detected as a function of the cutoff energy will allow tests of this hypothesis and constraints to be placed on the physical parameters of SNRs, in particular of the magnetic field strength. CTA offers the possibility of real breakthroughs in the understanding of cosmic rays; as there is the potential to directly observe their diffusion (see, e.g., [12]) The presence of a massive molecular cloud located in the proximity of a SNR (or any kind of CR accelerator) provides a thick target for CR hadronic interactions and thus enhances the gamma-ray emission. Hence, studies of molecular clouds in gamma-rays can be used to identify the sites whereCRsareaccelerated.Whiletravellingfromtheacceleratortothetarget, thespectrumofcosmicraysisastrongfunctionoftime,distancetothesource, and the (energy-dependent) diffusion coefficient. Depending on the values of these parameters varying proton, and therefore gamma-ray, spectra may be expected. CTA will allow the study of emission depending on these three quantities, which is impossible with current experiments. A determination, with high sensitivity, of spatially resolved gamma-ray sources related to the same accelerator would lead to the experimental determination of the local diffusion coefficient and/or the local injection spectrum of cosmic rays. Also, the observation of the penetration of cosmic rays into molecularclouds will be possible. If the diffusion coefficient inside a cloud is significantly smaller than the average in the neighbourhood, low energy cosmic rays cannot penetrate deep into the cloud, and part of the gamma-ray emission from the cloud is suppressed,withtheconsequencethatitsgamma-rayspectrumappearsharder than the cosmic-ray spectrum. Both of these effects are more pronounced in the denser central region of thecloud.Thus,withanangularresolutionoftheorderof≤1arcminonecould resolve the inner part of the clouds and measure the degree of penetration of cosmic rays.

The galactic centre region

It is clear that the galactic centre region itself will be one of the prime science targets for the next generation of VHE instruments [20, 21]. The galactic centre hosts the nearest super-massive black hole, as well as a variety of other objects likely to generate high-energy radiation, including hypothetical darkmatter particles which may annihilate and produce gamma-rays. Indeed, the galactic centre has been detected as a source of high-energy gamma-rays, and indicationsforhigh-energyparticlesdiffusingawayfromthecentralsourceand interactingwiththedensegascloudsinthecentralregionhavebeenobserved. In observations with improved sensitivity and resolution, the galactic centre canpotentiallyyieldavarietyofinterestingresultsonparticleaccelerationand gamma-ray production in the vicinity of black holes, on particle propagation in central molecular clouds, and, possibly, on the detection of dark matter annihilation or decay. The VHE gamma-ray view of the galactic centre region is dominated by two point sources, one coincident with a PWN inside SNR G0.9+0.1, and one coincident with the super-massive black hole Sgr A* and another putative PWN(G359.95-0.04).Aftersubtractionofthesesourcesdiffuseemissionalong the galactic centre ridge is visible, which shows two important features: it appears correlated with molecular clouds (as traced by the CS (1–0) line), and itexceedsbyafactorof3to9thegamma-rayemissionthatwouldbeproduced if the same target material was exposed to the cosmic-ray environment in our local neighbourhood. The striking correlation of diffuse gamma-ray emission with the density of molecular clouds within ∼150 pc of the galactic centre favours a scenario in which cosmic rays interact with the cloud material and produce gamma-rays via the decay of neutral pions. The differential gammaray flux is stronger and harder than expected from just “passive” exposure of the clouds to the average galactic cosmic ray flux, suggesting one or more nearby particle accelerators are present. In a first approach, the observed gamma-ray morphology can be explained by cosmic rays diffusing away from an accelerator near the galactic centre into the surroundings. Adopting a diffusion coefficient of D= O(1030) cm2/s, the lack of VHE gamma-ray emission beyond 150 pc in this model points to an accelerator age of no more
than 104 years. Clearly, improved sensitivity and angular resolution would permit the study of the diffusion process in great detail, including any possible energy dependence. An alternative explanation (which CTA will address) is the putative existence of a number of electron sources (e.g. PWNe) along the galactic centre ridge, correlated with the density of molecular clouds. Given the complexity and density of the source population in the galactic centre region, CTA’s improved sensitivity and angular resolution is needed to map the morphology of the diffuse emission, and to test its hadronic or leptonic origin. CTA will also measure VHE absorption in the interstellar radiation field (ISRF). This is impossible for other experiments, like Fermi-LAT, as their energy coverage is too small, and very hard or perhaps impossible for current air Cherenkov experiments, as they lack the required sensitivity. At 8 kpc distance, VHE gamma-ray attenuation due to the CMB is negligible for energies <500 TeV. But the attenuation due to the ISRF (which has a comparable number density at wavelengths 20–300 μm) can produce absorption at about 50TeV[22].Observationofthecutoffenergyfordifferentsourceswillprovide independenttestsandconstraintsofISRFmodels.CTAwillobservesourcesat different distances and thereby independently measure the absorption model and the ISRF. Due to their smaller distances there is less uncertainty in identifying intrinsic and extrinsic features in the spectrum than is the case for EBL studies.

Pulsar magnetospheres are known to act as efficient cosmic accelerators, yet there is no complete and accepted model for this acceleration mechanism, a process which involves electrodynamics with very high magnetic fields as well as the effects of general relativity. Pulsed gamma-ray emission allows the separation of processes occurring in the magnetosphere from the emission in the surrounding nebula. That pulsed emission at tens of GeV can be detected with Cherenkov telescopes was recently demonstrated by MAGIC with the Crab pulsar [28] (and the sensitivity for pulsars with known pulse frequency is nearly an order of magnitude higher than for standard sources). Current Fermi-LAT results provide some support for models in which gamma-ray emission occurs far out in the magnetosphere, with reduced magnetic field absorption (i.e. in outer gaps). In these models, exponential cut-offs in the spectral energy distribution are expected at a few GeV, which have already been found in several Fermi pulsars. To make further progress in understanding the emission mechanisms in pulsars it is necessary to study their radiation at extreme energies. In particular, the characteristics of pulsar emission in the GeV domain (currently best examined by the Fermi-LAT) and at VHE will tell us more about the electrodynamics within their magnetospheres. 

Studies of interactions of magnetosphericparticlewindswithexternalambient fields (magnetic, starlight, CMB) are equally vital. Between ∼10 GeV and ∼50 GeV (where the LAT performance is limited) CTA, with a special lowenergy trigger for pulsed sources, will allow a closer look at unidentified Fermi sources and deeper analysis of Fermi pulsar candidates. Above 50 GeV CTA will explore the most extreme energetic processes in millisecond pulsars. The VHE domain will be particularly important for the study of millisecond pulsars, very much as the HE domain (with Fermi) is for classical pulsars. On the other hand, the high-energy emission mechanism from magnetars is essentially unknown. For magnetars, we do not expect polar cap emission. Due to the large magnetic field, all high-energy photons would be absorbed if emittedclosetotheneutronstar,i.e.,CTAwouldbetestingouter-gapmodels, especially if large X-ray flares are accompanied by gamma-emission. CTA can study the GeV-TeV emission related to short-timescale pulsar phenomena, which is beyond the reach of currently working instruments. CTA can observe possible high-energy phenomena related to timing noise (in which the pulse phase and/or frequency of radio pulses drift stochastically) or to sudden increases in the pulse frequency (glitches) produced by apparent changes in the momentum of inertia of neutron stars. Periodicity measurements with satellite instruments, which require very long integration times, may be compromised by such glitches, while CTA, with its much larger detection area and correspondingly shorter measurement times, is not. A good compendium of the current status of this topic can be found in the proceedings and the talks presented at the “International Workshop on the High-Energy Emission from Pulsars and their Systems” .
. Such a violation of Lorentz invariance, on which the theory of special relativity is based, is present in some quantum gravity (QG) models. Burst-like events in which gamma-rays are produced, e.g. in active galaxies, allow this energy-dependent dispersion of gamma-rays to be probed and can beusedtoplacelimitsoncertainclassesofquantumgravityscenarios,andmay possibly lead to the discovery of effects associated with Planck-scale physics. CTA has the sensitivity to detect characteristic time-scales and QG effects inAGNlightcurves(ifindeedanyexist)onaroutinebasiswithoutexceptional source flux states and in small observing windows. CTA can resolve time scales as small as few seconds in AGN light curves and QG effects down to 10 s. Very good sensitivity at energies >1 TeV is especially important to probe the properties of QG effects at higher orders. Fermi recently presented results based on observations of a GRB which basically rule out linear-inenergy variations of the speed of light up to 1.2× the Planck scale [49] To test quadratic or higher order dependencies the sensitivity provided by CTA will be needed. This topic is thoroughly discussed in the book “Particle dark matter” edited by G. Bertone [46], and aspects of the fundamental physics implications of VHE gamma-ray observations are covered in a recent review.