Department of Physics, University of Namibia
Pvt Bag 13301, Windhoek, Namibia
Abstract. Several European institutions have successfully pioneered the principle of stereoscopic observation of gamma-ray induced air showers as a technique to do very high energy astronomy with the HEGRA instrument on the island of La Palma. Following the success of HEGRA, a next generation instrument, called HESS (High Energy Stereoscopic System), is currently under construction in the Khomas Highland of Namibia.
Sommaire. Plusieurs institutions européennes ont réussi à mettre au point le principe de l'observation stéréoscopique des gerbes produites dans l'atmosphère terrestre par les photons induits par les rayons gamma comme technique appliquée à l'astronomie des très hautes énergies avec l'instrument HEGRA sur l'Ile de La Palma aux Canaries. A la suite du succès d'HEGRA, un instrument de nouvelle génération, appelé HESS (Système Stéréoscopique à Haute Energie), est actuellement en construction dans l'Highland Khomas en Namibie.
This array of atmospheric Čerenkov detectors is intended to replace the older HEGRA (High Energy Gamma Ray Astronomy) project that is currently running on the island of La Palma in the Canary islands. As in the case of the HEGRA project, which is a collaboration involving MPIK and other institutions in Europe (University of Hamburg; University of Kiel; Complutense University of Madrid; Max Planck Institute for Physics, Munich; BUGH Wuppertal; Yerevan Physics Institute), HESS was from the onset also intended to be a collaboration, but on a larger scale yet.
The HEGRA project successfully proved the concept of stereoscopic observation of air showers produced by γ-ray photons entering the atmosphere from space. The ability to observe these showers with several telescopes at various viewing angles enabled the HEGRA instrument to determine the shower axis accurately, thus ensuring good angular resolution (0.1°) of the direction of motion of the incident γ-ray photon. The stereoscopic technique also enabled HEGRA to discriminate efficiently between γ-ray induced showers and showers induced by the nucleonic component of cosmic rays. An efficient triggering scheme also ensured a high degree of background suppression. This resulted in a low detection threshold in photon energy (500 GeV). Also, by using the redundant experimental data provided by several telescopes, researchers were able to calculate reliable energy spectra for γ-ray sources in space. For 1 TeV photons the HEGRA instrument can detect an energy flux as low as fE(> 1 TeV) = 10-12 erg/(cm2s), where fE= E d Fγ/d ln E = E2 d Fγ/dE.
The HESS array is being designed to be approximately one order of magnitude more sensitive than its predecessor, HEGRA.
This left southern Africa, with two possible sites: Sutherland in the Karoo in South Africa and the Gamsberg in the Khomas Highland of Namibia. Both these sites were also in contest for the SALT telescope and although Sutherland was chosen for this, the Gamsberg was identified as one of the best sites in the world.
Due to logistical reasons it was decided to situate HESS on a farm in the Gamsberg region, and not on the mountain itself. This farm (23°20'S, 415°50'E) is located about 100 km from Windhoek, the capital of Namibia. Although the site is not on the Gamsberg, it is still about 1800 m above sea-level, the same height as the highest point on the Sutherland site.
The proposed system will be able to detect γ-rays above a threshold photon-energy of about 40 GeV. However, due to the effect of the geomagnetic field on the charged secondary particles that produce the Čerenkov light flashes in the upper atmosphere, the operational threshold of the HESS instrument will have to be limited to photon energies above 100 GeV to ensure acceptable spatial resolution of the incident γ-ray photons (about 0.1° per photon).
Each IACT will have a field of view of about 5°. This generously large field of view will enable the mapping of extended γ-ray sources like supernova remnants (SNRs) and giant molecuclouds (GMCs).
A single IACT has the sensitivity to detect γ-ray sources with intensities
of about 20% of that of the Crab Nebula source, a frequently used standard.
In stereoscopic mode the HESS array will be able to detect sources at a
few percent of the Crab source, i.e., a new population of "milli-Crab"
sources will be available for discovery. For 1 TeV photons the minimum
detectable energy flux for the array will be in the order of fE
(> 1 TeV) = 10-13 erg/(cm2 s) for observation over
100 h (see Figure 1). This implies a flux sensitivity, Fγ(> 100
GeV) of 10-12 photons/ (cm2s) in 100 h.
During Phase II, the number of telescopes will be increased to 16 units, possibly making HESS the largest VHE array in the world. A possible lay-out of a 4-by-4 array is conceived. Also during Phase II an optical monitoring telescope (ATOM Automatic Telescope for Optical Monitoring) will be erected and will be slaved to the HESS array for multiwavelength observations. In addition, a LIDAR (Light Detection and Ranging - an instrument that uses a laser beam to probe the atmosphere) and other instruments to monitor atmospheric conditions above the site will be installed either late in Phase I or early in Phase II
For each individual telescope a segmented Davies-Cotton reflector (a spherical reflector) with a total area of 80 m2 and a focal length of 15 m will be used.
Each reflector will be made up of 300 circular aluminised glass mirror tiles, each with a diameter of 60 cm. A quartz coating protects the reflective layer. Two companies, COMPAS in the Czech Republic and GALACTICA in Armenia, are currently manufacturing the mirrors. Each of the individual mirror tiles will be automatically adjustable, using two actuators with a Hall effect sensor. The mirrors will be adjustable to a precision of 40 µm, corresponding to 1 mrad. The automatic mirror alignment procedure will use a CCD camera on the telescope camera lid, observing images of stars to do the calibration.
A 500-kg, 1.4-m diameter camera will be placed at the focus of this reflector. The whole structure will be supported by a space-frame to ensure stiffness and proper alignment. The mounting will be of the alt-azimuth type with freedom to move a full 360° in azimuth and from -30° to +180° in altitude. This freedom to have negative altitude settings will be used for camera maintenance and installation, as well as being the inactive position of the telescope. The alt-azimuth wheel friction drive will allow for positioning with a precision of 0.01°. The maximum drive speed will be 100° per minute.
The heart of the IACT is the imaging camera at the focus of the reflector. This camera will contain 960 so-called "smartpixels" (a single PMT with all the necessary electronics integrated in as a single replaceable unit), each with 0.16° field of view, arranged in a more or less circular way.
Each of these smartpixels contains a hexagonal Winston cone, a photomultiplier tube (PMT) with bialkali photocathodes and the relevant electronics. The high voltages for the PMTs are produced by cards containing a DC-DC converter at the back of the PMT. Also a large part of the triggering electronics, analogue signal storage and other electronics is situated either in the smartpixel or in the camera housing. This provides short paths for the fast analogue signals to allow short gate times in order to minimise the night sky background noise. The triggering is done by a first level trigger in each smartpixel, with a second level topological trigger in the camera housing itself. Depending on the conditions, the threshold for a single pixel is 3 to 5 photoelectrons. A third level global trigger is activated if a minimal number of telescopes have triggered within a short coincidence interval.
Perhaps one of the most pressing and long-standing problems that may be addressed with the HESS instrument is that of the origin of Cosmic Rays (CRs). Even now, more than 3 decades after the proposal of the theory of diffusive shock acceleration of charged particles in astrophysical shocks, the question of the sources of cosmic rays is not yet settled. Observations in the TeV spectrum may help to identify specific sources where cosmic rays are being accelerated. The prime candidates are Supernova Remnants (SNRs). Detection of γ rays from SNRs in the range of 100 GeV to 10 TeV will confirm that shock acceleration does indeed produce VHE particles at SNRs. In fact, γ-rays have been detected from SN 1006 by the CANGAROO IACT. γ rays produced by SNRs should be a combination of those produced by the nucleonic component and those produced by the leptonic component of CRs. The latter are produced by inverse Compton (IC) scattering on the ambient photon field and the first by π0 (which decays into γ rays) production through the interaction between CR nucleons and the Interstellar Medium (ISM).
This means that the γ fluxes from low density regions of the Galaxy should be dominated by photons produced by the leptonic component of CRs and those SNRs in high density regions should be dominated by photons produced by the hadronic component. At least 10 SNRs in the Sedov phase should be detectable by the instrument.
Another way to search for CR accelerators is by searching for Giant Molecular Clouds (GMCs) that are luminous in the TeV region. Among other possibilities, this could indicate the presence of an accelerator of high energy CR nucleons inside or near the GMC.
Another problem is the presence of TeV electrons in spectra measured at earth. Due to synchrotron and inverse Compton energy losses, the lifetime of electrons at these energies is very short. This implies that there must be (an) accelerator(s) nearby (within 100 pc). Prime candidates for electron accelerators are pulsar driven nebulae (Plerions). Electrons can be accelerated by the pulsar itself or at the pulsar wind termination shock in the nebula. These processes should also be visible in the TeV region due to inverse Compton and synchrotron self Compton processes.
Another problem is that of the very high energy (VHE) component of CRs of extragalactic origin. Shock acceleration cannot account for the acceleration of CRs up to energies E ≧ 1020 eV. Some theorists suggest that they may be the decay products of some massive particles from earlier epochs. These are sometimes called topological defects (TDs) like cosmic strings, monopoles, etc. that formed in a symmetry breaking phase transition in the very early universe. The possible collapse of these TDs may be just visible at the lower limit of the IACT's threshold energy.
Other sources to be searched for and known sources to be studied are accreting neutron stars and stellar black holes and the newly discovered superluminal objects, or microquasars discovered in our Galaxy. These are thought to be scaled down versions of Active Galactic Nuclei (AGNs) that are found in other galaxies. The centre of our own Galaxy also may hold surprises that can be detected in the TeV region, such as a large black hole.
Outside our own Galaxy, one of the most important classes of objects to be studied is AGNs. Already, two BL Lac (blazars) objects (Mrk501 and Mrk421) have been discovered to be sources of TeV γ rays. The synchronous flaring in the keV (X-ray) region and the TeV (γ-ray) energy regions supports the theory that both of these components are produced by synchrotron and synchrotron self Compton processes by the same relativistic electrons in jets ejected from a central object (possibly a giant black hole) in these galaxies. The IACT array should be able to detect several of these blazars. Other AGN class objects, like radio-loud galaxies and optically violent variable quasars, also produce VHE γ rays, and should also be detectable above 100 GeV. In addition to this, VHE radiation is also expected from AGNs without jet-like features, like Seyfert galaxies.
Additionally, VHE studies of rich clusters of galaxies should be informative about galaxy formation in the early universe.
From the field of observational cosmology, the observation of pair halos is of interest. Theory suggests that γ rays from UHE (Ultra High Energy) sources can be scattered on the 2.7 K microwave, infrared or optical Diffuse Extragalactic Background Radiation (DEBRA) fields. The photon-photon reactions then produce a cascade, and if the magnetic field near (within a few Mpc) the UHE γ-ray source is large enough, the electron/ positron pairs in the cascades will be isotropised. These electrons will then produce observable VHE γ-ray photons through inverse Compton scattering on the 2.7 K microwave DEBRA field. The discovery and mapping of these pair halos that are of distinct extragalactic origin will enable HESS researchers to determine several things.
Firstly, observing pair halos at different redshifts (different distances) will tell us something about the time evolution of the DEBRA fields. Secondly, comparison of the characteristic physical sizes of such pair halos with their redshift-distance relation (Hubble's law), will give us direct information on source distances without resorting to a distance-ladder technique.
Also of cosmological interest is the issue of γ rays from dark matter, especially from massive relic particles produced in the very early universe, such as WIMPs (Weakly Interacting Massive Particles). A specific experiment in this regard was suggested at a HESS workshop in December 1999, concerning the decay of neutralinos in the Galactic halo. Theoretical models can predict the difference in the VHE energy spectra from the Galactic halo with and without neutralino decay. By measuring the energy spectrum of VHE radiation from the galactic halo, HESS should be able to provide clues in this regard.
The above discussion broadly illustrates only some of the possibilities of HESS science. It must be stressed that there may be many more possibilities that cannot be discussed here and/or that the author is not aware of, nor capable of discussing at this stage.