Physics and Astrophysics of High Energy Nonthermal Objects

  • Blazars and Radiogalaxies
  • Clusters Of Galaxies
  • Convertor Mechanism And Off-axis Emission
  • Diffusive Shock Acceleration
  • Extragalactic Background Light
  • Gamma Ray Binaries
  • Hydrodynamics
  • Interstellar Medium
  • Molecular Clouds
  • Prompt Afterglows Of Gamma Ray Bursts with ROTSE
  • Pulsar Wind Nebulae / Plerions
  • Supernova Remnants
  • Ultra High Energy Cosmic Rays and Neutrinos

    Imaging Atmospheric Cherenkov Technique

  • CTA
  • HEGRA, HESS, 5@5...


    Blazars And Radiogalaxies

    Blazars are thought to be radiogalaxies, i.e. radio-loud Active Galactic Nuclei (AGN), whose relativistic jet is pointing close to the line of sight. Their spectral energy distribution (SED) is dominated by two highly variable, non-thermal radiation components peaking at low (IR-to-X-ray) and high (MeV-to-TeV) energies. While the low-energy peak is commonly explained as synchrotron emission from relativistic electrons, how these are accelerated and the origin of the high-energy peak remain a major research issue, as well as the launch, collimation and acceleration of the relativistic jet. To this respect, the study of the correlated spectral and flux variability in different energy bands represents a fundamental diagnostic tool. The activity of our group has mainly concentrated on BL Lacs, a sub-class of blazars which represent now a well established population of TeV gamma-ray emitters and have shown the most rapid variability and widest spectral changes (e.g. Mkn 501 and PKS 2155-304), and on nearby radiogalaxies, as possible gamma-ray sources (as confirmed by HEGRA and HESS with M87, and by EGRET with Centaurus A). Our main research topics are (1) multiwavelength properties, with emphasis on X-ray and TeV bands, comparison and modelling of the different SEDs, tests of the "blazar sequence" scenarios; (2) jet emission mechanisms, both at sub-pc and kpc scales, leptonic and hadronic scenarios for particle acceleration and radiation, TeV gamma-ray production mechanisms in radiogalaxies. Our group has proposed and organized several observations and multiwavelength campaigns with TeV (HEGRA, HESS) and X-ray (RXTE, XMM, CHANDRA, Swift, Suzaku) telescopes, and has a leading role in the proposal, planning and interpretation of the AGN program in the HEGRA and HESS Collaborations.


    Clusters Of Galaxies

    Diffuse synchrotron radio halos are observed in a growing number of rich clusters of galaxies. This tells us that a population of relativistic electrons and a microGauss magnetic field are present in the intracluster medium. Even if the existence of relativistic electrons is firmly proved by observations, the origin of these particles and the mechanisms through which they are accelerated are still unknown. Moreover, since the majority of the acceleration mechanisms at work in astrophysical sources accelerate protons as well, it is generally assumed that a hadronic cosmic ray component will also be present in the intracluster medium. From X-ray observations we also know that clusters contain a considerable amount of baryons in the form of a hot diffuse gas. These facts led to the idea that clusters might be gamma ray sources, due to the decay of neutral pions produced in the inelastic interactions between the cosmic ray protons and the intracluster medium. This possibility was extensively studied after the discovery that relativistic protons remain diffusively confined within the cluster volume for cosmological times, without losing their energy. As a consequence, both the probability of having inelastic proton proton collisions and the related gamma ray emissivity are expected to be enhanced. Other contributions to the total gamma ray emission come from inverse Compton and Bremsstrahlung emission from relativistic electrons. All these facts make clusters of galaxies ideal targets for gamma ray telescopes operating both in the GeV and TeV energy domain.


    Convertor Mechanism And Off-axis Emission

    There are several viable mechanisms of particle acceleration related to relativistic outflows, e.g. through the relativistic shocks or reconnection of the magnetic field. In radiation-dominated environments they proceed in rather unusual ways. This concerns, in particular, the acceleration of electrons within high density radiation fields. Generally, fast inverse Compton losses should prevent efficient acceleration of electrons, but proceeding in Klein-Nishina regime, they can create a rather special environment, where the standard diffusive shock acceleration can be outperformed by convertor mechanism. This leads not only to more efficient acceleration, but creates a strongly anisotropic distribution of accelerated particles. This would consequently have a strong impact on the predicted spectra and fluxes of astrophysical sources, especially at high (GeV-TeV energies), which appears to be much harder compared to the predictions derived from the standard Doppler boosting considerations alone (so called off-axis emission).


    Diffusive Shock Acceleration

    Diffusive shock acceleration is thought to be responsible for acceleration of cosmic rays in several astrophysical environments. Despite the success of this theory, some issues are still a subject of much debate for the theoretical and phenomenological implications that they may have. One of the most important of these is the reaction of the accelerated particles on the shock: the violation of the test particle approximation occurs when the acceleration process becomes sufficiently efficient that the pressure of the accelerated particles is comparable with the incoming gas kinetic pressure. Both the spectrum of the particles and the structure of the shock are changed by this phenomenon, which is therefore intrinsically nonlinear. Another line of research of the group concerns the acceleration of particles at shocks with the presence of strong radiative losses. Since shocks are ubiquitous in astrophysical environments, results from this research may find applications in the modelling of a great variety of objects (e.g. supernova remnants, clusters of galaxies).

    Extragalactic Background Light

    The diffuse Extragalactic Background Light (EBL) consists of the sum of the light produced by all extragalactic sources over cosmic time. The Optical to far-IR range is particularly important, since the starlight and the dust emission from galaxies over a wide range of redshifts falls in this energy band, thus providing information about the epochs of formation and the history of evolution of galaxies. However, direct measurements are very difficult, due to the presence of significantly brighter foregrounds (light scattered or emitted by dust in the solar system, starlight and dust emission from our own galaxy). Gamma-rays from 0.1 to 100 TeV offer an independent (and in certain wavebands the only) way to probe this diffuse field. TeV gamma-rays interact with EBL photons through the pair-creation process (γ γ --> e+e-). The resulting optical depth is strongly energy dependent, according to the spectrum of the EBL. Therefore it is possible to extract information on the EBL at different wavelengths through the detection and identification of the EBL absorption features in the TeV spectra of objects at known redshift. Our group is specialized in the analysis and correct interpretation of these features, in particular for BLLacs, which require the simultaneous analysis of their multiwavelength properties and emission mechanisms in order to try to disentangle absorption from intrinsic features. The power of this technique has been demonstrated by the several fundamental constraints on the EBL derived from the TeV spectra of Mkn 421, Mkn 501, 1ES 1426+428, 1ES 1101-232 and H2356-304.

    Gamma Ray Binaries

    Compact binary systems formed by a normal star and a compact object, either a pulsar, a normal neutron star or a black-hole, have been detected at TeV energies. Although it is still unclear what are the details of the physics behind such very energetic radiation, it is unavoidable that very efficient particle acceleration, strong radiation fields, particle transport and magnetic fields play an important role in the high energy processes that produce the very high energy photons. In such a context, it is of great importance to explore the possible physical conditions that could take place in compact binaries. Namely, it seems necessary to simplify in a suitable manner the complex scenario that may realize in these objects, mainly linked to relativistic outflows, such that a meaningful comparison can be carried out between theoretical predictions and observational data. An accurate and solid modeling of these sources, in combination with good quality TeV data plus information at lower energies, can allow to infer information about the particle nature, the radiation, matter and magnetic fields, or the acceleration and radiation mechanisms, thereby constraining important properties of the emitter.



    Multiwavelength observations show that relativistic outflows related to both galactic and extragalactic sources are sites of efficient particles acceleration and nonthermal radiation. The study of the acceleration and radiation processes that take place in these objects should be coupled with selfconsistent magnetohydrodynamical description. Currently we are focused on the study of MHD processes in closed binary systems.

    Interstellar Medium

    The space between the stars in Galaxies contains gas, dust particles, magnetic fields, and relativistic electrons and nuclei (cosmic rays). These different components are strongly coupled together and are therefore described as a single dynamical entity called the interstellar medium (ISM). The ISM, which strongly influences Galactic evolution, has been extensively studied at all wavelengths. In particular radio emission from atomic hydrogen traces the distribution of the most common atomic gas, whilst submillimetre emission from CO and other molecules can be used as a tracer of molecular hydrogen given an abundance ratio, X. During their travel through interstellar space the relativistic cosmic rays, both electrons and protons, encounter nuclei and produce diffuse gamma ray emission. The gamma ray emissivity is proportional to the product of the cosmic ray flux and matter density. The cosmic ray flux, which is likely to vary in the different locations of the Galaxy, was until now measured only in the vicinity of the Sun. Therefore the gamma ray emission provides a unique probe of the cosmic ray flux if the matter density is known. Alternatively, the knowledge of the gamma ray emission and of the cosmic ray flux in the Galaxy provides the calibration of the conversion factor X. The distribution of ISM material and the influence of magnetic fields on it (which is partly determined by ionisation due to cosmic ray bombardment) constitute the initial conditions for star formation and is therefore important for studies of star formation itself and for galaxy evolution. Robust theoretical tools are needed to combine the new data from the NANTEN high sensitivity mapping of Galactic molecular hydrogen and the soon-to-be launched telescopes, AGILE and GLAST. For this reason we are developing a model, which instead of considering the poorly-known global quantities in the Galaxy, describes the diffuse Galactic gamma ray emission region by region. This model can fruitfully take advantage of the ISM and gamma ray data in order to make solid predictions concerning the cosmic ray flux and to unveil target-accelerator systems in our Galaxy.

    Molecular Clouds

    The observed diffuse gamma ray emission from the galactic plane is believed to be dominated by the decay of neutral pions produced during inelastic collisions of cosmic rays with interstellar gas. If, as a first order approximation, one assumes that the cosmic ray spectrum is the same everywhere in the Galaxy, then the gamma ray emission is expected to be simply proportional to the gas column density. As a consequence, when the line of sight of an observer intersects regions of enhanced density, such as giant molecular clouds, the gamma ray emission is expected to be correspondingly enhanced. The importance of the detection of molecular clouds in gamma rays is widely recognized, especially in relation to the problem of the origin of cosmic rays. Molecular clouds located in the vicinity of cosmic ray accelerators could provide a dense target for cosmic ray interactions, leading to an amplification of the the gamma ray emission from the region, making easier the identification of cosmic ray sources. On the other hand, even in the absence of an accelerator, molecular clouds embedded in the ``sea'' of galactic cosmic rays are expected to emit gamma rays. If cosmic rays can freely penetrate the clouds, the high energy gamma ray spectrum is expected to mimic the slope of the cosmic ray spectrum, with the total gamma ray luminosity depending only on the total mass of the cloud. For this reason, molecular clouds can be used to probe the cosmic ray energy spectrum and its absolute flux in different parts of the Galaxy. If the diffusion coefficient inside the cloud is significantly small compared to that derived for the galactic disk, the observed gamma ray spectrum will appear harder than the cosmic ray spectrum, mainly due to the slower penetration of the low energy particles towards the core of the cloud. This may produce a great variety of gamma ray spectra.


    Prompt Afterglows Of Gamma Ray Bursts With ROTSE

    Our group is an official member of the ROTSE collaboration (F.Aharonian as Co-PI). The aim of the collaboration is to study the early or prompt optical afterglow within seconds of their detection at gamma-ray energies, using a network of four robotic 0.45m diameter optical telescopes with 1.8o × 1.8o field of view over 4 Megapixel CCD camera. One of these telescopes is located at the H.E.S.S. site in Namibia. In spite of some other similar projects, so far prompt afterglows have been detected only by the ROTSE telescopes. With the launch of the Swift gamma-ray burst explorer in late 2004 great success has been made in study of early afterglow phase of GRBs. Recently the ROTSE-IIIc telescope at the H.E.S.S. site, Namibia, obtained the earliest detection of optical emission from a Gamma-Ray Burst (GRB), beginning only 21.8 s from the onset of Swift GRB 050801. The most densely sampled yet early lightcurves reveal unexpected behavior. They do not fade or brighten significantly over the first ~250 s, after which there is an achromatic break and the lightcurve declines in a typical power-law fashion. The Swift/XRT also obtained early observations starting at 69 s after the burst onset. The X-ray lightcurve shows the same features as the optical lightcurve. These correlated variations in the early optical and X-ray emission imply a common origin in space and time. This behavior is difficult to reconcile with the early theoretical predictions and challenges the standard models of early afterglow emission. Finally we note that ROTSE-III is fulfilling its potential for GRB science, and provides optical observations for a variety of astrophysical sources in the interim between GRB events.

    Pulsar Wind Nebulae / Plerions

    Although, the radiation of pulsar wind nebulae are readily explained by the standard model, which assumes acceleration of ultrarelativist electrons by the pulsar wind termination shock, in some of these systems nonthermal radiation could be produced by high energy hadrons. This radiation is expected to be produced via inelastic interactons with the ambient medium. In this regard the extended TeV source associated with the pulsar PSR B0833-45 (Vela~X) could be a good candidate for such a "hadronic plerion". To explore this possible scenario, we work on calculation of the high energy emission due to nucleonic processes, including the synchrotron and IC emission from primary and secondary electrons. The TeV neutrino production is an important feature of this model, and our calculations show that Vela X could be one of the best known candidates for detection of neutrinos with the next generation km3 scale telescopes. Another very special case of plerions is pulsars located in radiation dominated enviroments, such as that found in the Galactic Center or (even more specific situation) in binary systems. Fast Inverse Compton cooling in the Klein-Nishina regime may result in radically different electron spectra than those formed via synchrotron cooling. We explored these effects and their impact on the X-ray and gamma-ray spectra produced in electron accelerators in the central ≤ 10 pc region in comparison to elsewhere in our galaxy. This region appears to be the only location in our Galaxy in which PWNe with high magnetic fields and moderate spin-down luminosities can produce detectable gamma-ray emission.


    Supernova Remnants

    In 1934, Baade and Zwicky first proposed that supernovae are the sources of galactic cosmic rays (CR). To support their idea, they used a simple argument: the observed CR population can be maintained at the present level if a small fraction (a few percent) of the galactic supernovae kinetic energy is somehow converted into CR. This argument is strengthened by the fact that it is commonly believed that CR can be efficiently accelerated via the Fermi mechanism at shock waves that form during the expansion of supernova remnant in the interstellar medium. The acceleration of CR in supernova remnants must be accompanied by a copious gamma ray emission due to the decay of neutral pions produced in interactions between relativistic protons and protons in the interstellar medium. Because the energy transferred to accelerated particles is tightly constrained by the observed total CR power, it is possible to obtain an almost model independent prediction of the gamma ray luminosity of supernova remnants. Recently, the HESS telescope detected a few supernova remnants at TeV energies, whose flux level matches very well the above mentioned predictions. Though the HESS results undoubtedly constitute one of the most important advancements in the field, they still do not provide us with a definite and direct evidence of proton acceleration at supernova remnants. In fact, competing leptonic processes can also generate gamma rays, and thus an accurate modelling is needed in order to disentangle the different contributions. Also neutrinos are produced during the hadronic interactions responsible for the generation of gamma rays. Their detection, though challenging even for the next generation of telescopes, would constitute an unambiguous proof for proton acceleration in these objects.


    Ultra High Energy Cosmic Rays And Neutrinos

    Ultra high energy cosmic rays consist of proton and nuclei with energies in excess of 1018 eV which arrive at Earth after having propagated over Mpc distances or more (ie. periods of time greater than Myr). The sources of these high energy particles is presently unclear, though constraints can be placed on the source region through arguments pertaining to the magnetic field strength required to confine the cosmic ray to the accelerating region for long enough for the particles to reach these high energies, and from a contraint on the maximum source distance resulting from the fact that the Universe is not opaque to the propagation of these high energy particles. Such energetic cosmic rays, however, are able to leave the confines of the microGauss Galactic magnetic field, and propagate through the extragalactic background radiation fields. The dominant radiation fields present in the Universe consist of the microwave background, a relic of the Universe's denser hotter past, and the infra-red background, an accumulation of the star light produced since the stars first turned on a redshift of 12 (ie. 400 Myr after the big bang), and a redistribution of this starlight by dust which re-emits a fraction of this light in longer wavelengths. As the ultra high energy cosmic rays propagate through these radiation fields they interact with the photons and lose energy, predominantly through the generation of electron positron pairs and pions. Ultra high energy cosmic ray protons eventually interact with this light, which to the protons (thanks to the effects of relativity) has an energy of 108 eV, ie. an energy sufficiently high (and a wavelength sufficiently small) to excite the quark structure of the proton, leading to the production of ultra high energy neutrinos and photons. These neutrinos, after production, will seldom interact with other particles, requiring large densities and volumes (such as the Earth) to do so due to their small interaction cross section. Consequently the ultra high energy neutrinos resulting from these interactions, anticipated to be detected at Earth by kilometer size detectors such as IceCube and KM3NET, are dependent on neutrinos produced through the entire age of the universe. This perhaps hilights the perculiar connection between the small scale world of elementary particles and that of the largest scale, the cosmos.


    A significant part of our groups activity is related to the study of the theoretical aspects of ground-based gamma-ray experiments with an emphasis on stereoscopic systems of imaging atmospheric Cherenkov telescopes (IACTs). This work includes (1) the formulation of astrophysical motivations, and corresponding requirements on the detectors based on modeling of different astrophysical source populations (2) the study of the potential of IACT arrays based on detailed Monte Carlo simulations.
    Our group has the status of being the second official group representing MPIK in the HESS collaboration. Therefore, a significant fraction of our groups work is related to the HESS experiment which includes observations, data reduction and interpretation. Members of the group actively participate in the preparation of HESS papers and were the coordinators (corresponding authors) of several of them.
    Members of the group have been involved also in activity related to the future (beyond HESS) stereoscopic imaging atmospheric Cherenkov telescope (IACT) arrays with emphasis on the theoretical aspects of ground-based gamma-ray experiments, in particular on formulations of basic requirements to the future ground-based detectors - basic scientific motivations, choice of the energy threshold etc.

    CTA Science case proposal

    Most of the members of the high-energy astrophysics group, in collaboration with other members of the division on particle physics and astroparticle physics, has been actively involved in the proposal of different Science cases that could be studied by the future instrument Cerenkov Telescope Array, or CTA. The fields touched by the CTA Science case proposal are the Galactic Center (Christopher van Eldik & Daniil Nekrassov), the Galactic diffuse emission (Sabrina Casanova, Kathrin Egberts & Andrew Taylor), Microquasars/Binaries (Valenti Bosch-Ramon, Dmitry Khangulyan & Anja Szostek), SNR (Matthieu Renaud & Omar Tibolla), Clouds (Stefano Gabici), Unidentified Sources (Karl Kosack & Gerd Puhlhofer), Extragalactic Sources (AGN), Blazars (Frank Rieger, Dmitry Khangulyan & Martin Raue), Clusters (Wilfried Domainko), and the Composition of Cosmic rays (Andrew Taylor, Rolf Buehler & Kathrin Egberts).

    The document with the complete Science case proposal can be found here.


    TeV Gamma-ray Astronomy with HEGRA

    In 1992 the HEGRA collaboration decided to upgrade the original version of the "HEGRA IACT System" project, originally proposed in 1990 by F.A. Aharonian and O.C. Allkofer. The contribution of our newly formed group to this upgrade was to make recommendations concerning the requirements on new imaging cameras for the stereoscopic system, and to calculate the performance of the telescope system, based on Monte Carlo simulations. The subsequent experimental/methodological results obtained by the HEGRA group with the stereoscopic system generally confirmed the early theoretical expectations [see F.A. Aharonian F.A. (HEGRA collaboration): The Project of the HEGRA Imaging Cherenkov Telescope System: Status and Motivations, in: Lamb R.C. (ed.) ``Towards a Major Atmospheric Cherenkov Detector - II'' (Calgary), 1993, pp. 81-86 ].



    In the mid 1990s, the concept of stereoscopic arrays of 10m diameter class imaging Cherenkov telescopes observing Cherenkov light of air showers simultaneously at different angles, was recognized as the most promising approach to dramatically improve the sensitivity and push the detection threshold down to 100 GeV [see F.A. Aharonian, C.W. Akerlof, Annu. Rev. Nucl. Part. Sci. 47, 273, 1997 ]. In 1997, F.A. Aharonian, H.J. Voelk, and W. Hofmann from the division of Particle Physics wrote the original H.E.S.S. Letter of Intent . Two joint papers, written together with A. K. Konopelko, formed the physical basis for the description of the instrument characteristics. In joint visits to Calar Alto and Namibia the final experiment site in Namibia was selected, and the necessary contracts were concluded in cooperation with the Generalverwaltung of the MPG. H.E.S.S., an array of four 13m-diameter IACTs equipped with a large ∼ 5o FoV multi (almost 1000) pixel cameras, was completed in 2004. It covers a broad energy band from 100 GeV to 100 TeV with an angular resolution of a few arcminutes and minimum detectable energy flux approaching 10-13 erg cm-2s-1. Although the power of the stereoscopic approach was convinsingly demonstrated by the HEGRA system it was the H.E.S.S. that elevated the status of the field to that of a truly astronomical discipline.


    Next Generation of IACT arrays

    Planning the next generation of IACT arrays has two major objectives: (i) an order of magnitude improvement of the flux sensitivity in the standard (0.1 to 10 TeV) energy regime and (ii) an agressive expansion of the energy domain of IACT arrays in both directions, down to 10 GeV and up to 1 PeV (see [1], [2], [3]).

    5@5 as a concept for a "Gamma-Ray Timing Explorer"

    Many of the GeV gamma-ray sources (E ≥ 0.1 GeV) observed by EGRET may be different from TeV sources detectable at (E ≥ 0.1 TeV) with an instrument like H.E.S.S. However the proximity of the intermediate range below 100 GeV to the energy range covered by EGRET suggests that many objects established as GeV emitters have a good chance to be detected in such an intermediate range - and then also or even better by ground-based instruments! The question is how close one must come to 1 GeV. Although the two largest gamma-ray source populations identified by EGRET - the radiopulsars and distant AGN - do not show a significant steepening or cutoff up to 10 GeV, the theoretical studies of gamma-ray production and absorption conditions in these objects, as well as rather general phenomenological considerations predict cutoffs in the energy spectra around 10 GeV or less. In addition, for any reasonable model of the diffuse extragalactic cosmic background radiation, we should expect sharp cutoffs in the spectra of distant extragalactic objects with redshift z∼1 at energies as low as 30 GeV. This implies that for the study of cosmologically distant sources, like the GeV blazars discovered by EGRET or possible GeV components of Gamma Ray Bursts (GRBs), the energy threshold of the detectors should be less than 10 GeV at which energy the Universe is most likely transparent up to at least z≅3. An instrument like GLAST, operating effectively in the 0.1 to 10 GeV energy region, nicely suits this task "from below". In particular, it is expected that GLAST will detect several thousand of AGN. On the other hand, the relatively small detection area Aeff ≅ 0.8 m2 of GLAST limits the potential of this instrument for detailed studies of the temporal and spectral characteristics of highly variable gamma-ray sources like blazars, which have variability timescales less than a few hours, or of solitary events like GRBs with a duration of 10-2 to 103 seconds. In this regard, GLAST can hardly match the performance of current X-ray detectors that have similar detection areas but operate in a regime of photon fluxes that exceed the fluxes of MeV/GeV gamma-rays by many orders of magnitude.
    The idea of a "fast" gamma-ray detector to study transient gamma-ray phenomena with an adequate photon detection rate motivated to a large extent our study of a possible extension of the ground-based Cherenkov technique, with its huge detection area of 104 to 105 m2, "from above" down to energies of several GeV.
    We have called such an instrument 5@5 - a 5 GeV energy threshold array of imaging atmospheric Cherenkov telescopes at 5 km altitude. With its potential to detect typical EGRET gamma-ray sources with spectra extending beyond several GeV, during exposure times from 1 to 103 seconds, such a detector may serve as an ideal "Gamma-Ray Timing Explorer" for the study of transient non-thermal phenomena like gamma-radiation from AGN jets, synchrotron flares of microquasars, the high energy (GeV) counterparts of Gamma Ray Bursts, etc. The 5@5 concept is complementary to that of GLAST which will be the most powerful instrument for the study of persistent GeV gamma-ray sources to about 30 GeV. The existing technological achievements in the design and construction of fine grained (typically 1000 pixels) high resolution imagers, as well as of large 20 to 30m diameter class multi-mirror dishes with rather modest optical requirements, should permit the construction of such a detector in the not too distant future. An ideal site for such an instrument would be a high-altitude, 5 km a.s.l or more, flat area with a linear scale of about 100 m in a very arid mountain region adjacent to the ALMA site in the Atacama desert of Northern Chile.

    Probing the 10 to 100 TeV range by IACT arrays

    Despite intensive efforts in the early 90s that were motivated by claims about the detection of TeV/PeV gamma-rays from Cyg X-3, all-sky surveys by the most sensitive air shower particle detectors like CASA/MIA, Cygnus, EAS TOP, and HEGRA have failed to detect sources of ultra high energy (UHE), E ≥ 100 TeV, gamma-rays at a flux level F ≥ 10-14 cm-2s-1. As a result there is only little interest in the energy range above several tens of TeV at present. Instead we observe an interesting trend of reducing the energy threshold of the particle arrays down to several TeV by using large water Cherenkov detectors like MILAGRO, or dense air shower arrays installed at high mountain altitudes (TIBET Array). However, the energy region above 10 TeV is of high astrophysical interest. For example, the detection of 10 to 100 TeV gamma-rays from shell type SNRs would provide an important evidence for the expectation that SN shock fronts accelerate protons up to energies between 100 and 1000 TeV. Another important class of targets would be pulsar driven nebulae. Observations of the Crab Nebula and Vela X with the HEGRA and HESS telescope arrays demonstrate that gamma-ray spectra of these objects extend well beyond 10 TeV. This could be interpreted as acceleration of electrons with maximum possible acceleration rate or a hint for the acceleration of protons and nuclei up to 1 PeV. This would not only require a major revision of the models of particle acceleration in pulsar magnetospheres or by pulsar winds, but would also imply a significant role of pulsars as contributors to the nucleonic component of galactic cosmic rays.
    Another interesting issue is connected with observations of gamma-rays with energies ≥ 10 TeV from relatively nearby extragalactic sources, for example from the radiogalaxies M87 (15 Mpc) or Cen A (3.5 Mpc) that are potential sources of VHE gamma-radiation. The search for absorption features in the gamma-ray spectra of these objects between 20 and 100 TeV could provide important information about the diffuse extragalactic infrared background at wavelengths λ ≥ 100 μ.
    A straightforward approach in probing the Multi-TeV region would be the use of arrays of IACTs with rather modest mirror areas (10 m2 or so) but equipped with multichannel cameras with a wide (6o - 9o) FoV and separated from each other by distances from 250 m to 500 m. The simulations made for an array of such IACTs consisting of 4 to 25 telescopes show that besides extraordinarily large collection areas (exceeding 1 km2) such 10 TeV threshold arrays can provide excellent angular resolution (within a few arcminutes), good energy resolution (≤ 20 per cent), and a rather effective rejection (better than a factor of 10) of hadronic showers. Such an array could be also an effective detector for the study of the energy spectrum of cosmic ray protons, and perhaps also of particles from the Fe-group at energies up to the "knee" around 1 PeV.