Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using 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 the 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.

A significant fraction of the energy density of the interstellar medium is in the form of highenergy charged particles (cosmic rays) 1 . The origin of these particles remains uncertain.Although it is generally accepted that the only sources capable of supplying the energy required to accelerate the bulk of Galactic cosmic rays are supernova explosions, and even though the mechanism of particle acceleration in expanding supernova remnant (SNR) shocks is thought to be well understood theoretically 2,3 , unequivocal evidence for the production of high-energy particles in supernova shells has proven remarkably hard to find. Here we report on observations of the SNR RX J1713.7−3946 (G347.3−0.5), which was discovered by ROSAT 4 in the X-ray spectrum and later claimed as a source of high-energy γ-rays 5,6 of TeV energies (1 TeV=10 12 eV). We present a TeV γ-ray image of the SNR: the spatially resolved remnant has a shell morphology similar to that seen in X-rays, which demonstrates that veryhigh-energy particles are accelerated there. The energy spectrum indicates efficient acceleration of charged particles to energies beyond 100 TeV, consistent with current ideas of particle acceleration in young SNR shocks. RX J1713.7−3946, together with several other southern hemisphere SNRs, is a prime target for observations with the High Energy Stereoscopic System (H.E.S.S.), a new system of four imaging atmospheric Cherenkov telescopes located in the Khomas Highland of Namibia.H.E.S.S. 7,8 (we note that V. F. Hess discovered cosmic rays) exploits the most effective detection technique for very-high-energy γ-rays, namely, the imaging of Cherenkov light from air showers. This technique, which was pioneered by the Whipple collaboration 9 , makes use of the fact that whenever a high-energy γ-ray hits the Earth's atmosphere it is absorbed and initiates a cascade of interactions with air atoms, leading to the formation of a shower of secondary charged particles.Those travelling faster than the local speed of light in air emit Cherenkov radiation, which results in a brief flash of blue Cherenkov light detectable at ground level. By using a telescope with sufficient mirror area to collect enough of the faint light signal, and a fast camera with fine pixelation, one can image the shower and reconstruct from this image the direction and energy of the primary γ-ray.Combined with the approach of stereoscopic imaging of the cascade using a system of telescopes, as pioneered by the HEGRA collaboration 10 , this yields a very powerful technique for imaging and obtaining energy spectra of astronomical sources at TeV energies.The H.E.S.S. experiment is such a stereoscopic system that consists of four 13-m-diameter telescopes 11 spaced at the corners of a square of side 120 m, each equipped with a 960-phototube camera 12 covering a large field of view of diameter 5°. Construction of the telescope system started in 2001; the full array was completed in December 2003 with the commissioning of the fourth telescope. HESS has an angular resolution of ...

Abstract. The high-frequency peaked BL Lac PKS 2155−304 at redshift z = 0.117 has been detected with high significance (∼45σ) at energies greater than 160 GeV, using the H.E.S.S. stereoscopic array of imaging air-Cherenkov telescopes in Namibia. A strong signal is found in each of the data sets corresponding to the dark periods of July and October, 2002, and June-September, 2003. The observed flux of VHE gamma rays shows variability on time scales of months, days, and hours. The monthly-averaged integral flux above 300 GeV varies between 10% and 60% of the flux observed from the Crab Nebula. Energy spectra are measured for these individual periods of data taking and are characterized by a steep power law with a time-averaged photon index of Γ = 3.32 ± 0.06. An improved χ 2 per degree of freedom is found when either a power law with an exponential cutoff energy or a broken power law are fit to the time-averaged energy spectrum. However, the significance of the improvement is marginal (∼2σ). The suggested presence of features in the energy spectrum may be intrinsic to the emission from the blazar, or an indication of absorption of TeV gamma rays by the extragalactic infrared background light.

Very high energy γ-rays probe the long-standing mystery of the origin of cosmic rays. Produced in the interactions of accelerated particles in astrophysical objects, they can be used to image cosmic particle accelerators. A first sensitive survey of the inner part of the Milky Way with the High Energy Stereoscopic System (HESS) reveals a population of eight previously unknown firmly detected sources of very high energy γ-rays. At least two have no known radio or x-ray counterpart and may be representative of a new class of “dark” nucleonic cosmic ray sources.

A sample of 54 6.7-GHz methanol masers was monitored using the Hartebeesthoek 26-m telescope during the period 1999 January -2003 March. The observations were taken at 1-2 week intervals, with daily observations when possible if a source was seen to be varying rapidly. It was found that the majority of the sources display a significant level of variability. The timerange of variations range from a few days up to several years. The types of behaviour observed included non-varying, monotonic increases or decreases, as well as aperiodic, quasi-periodic and periodic variations. Seven sources show clear evidence of periodicity, with periods ranging from 132 d up to 520 d.Seven sources, viz. G188.95+0.89, 196.45-1.68, G328.237-0.548, G331.13-0.24, G338.92-0.06, G339.62-0.12 and G9.62+0.20, show strong evidence of periodicity. Rigorous analysis of the periodicity is beyond the scope of this paper and will be presented in Goedhart, Gaylard & van der Walt (in preparation). The following sections describe the variability seen in the entire source sample. Variability indexEach maser spectrum consists of a number of peaks, corresponding to emission from maser spots at different velocities. Channels with no maser emission show the maxima and minima of the noise in the spectra. Velocity channels at the maximum of each peak were selected for the following analysis. Because the relation of the spatial structure of the maser to the velocity structure is not known in many cases, it was decided not to average the channels in each peak (to C 2004 RAS, MNRAS 355, 553-584Long-term monitoring of 6.7-GHz methanol masers 555