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.

Abstract.We report the detection of a point-like source of very high energy (VHE) γ-rays coincident within 1 of Sgr A * , obtained with the HESS array of Cherenkov telescopes. The γ-rays exhibit a power-law energy spectrum with a spectral index of −2.2 ± 0.09 ± 0.15 and a flux above the 165 GeV threshold of (1.82 ± 0.22) × 10 −7 m −2 s −1 . The measured flux and spectrum differ substantially from recent results reported in particular by the CANGAROO collaboration.

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